THE HYGIENE OF THE ZONES

Introduction: Some General Relations of Climate and Health—A Complex Subject—Climate, Micro-organisms and Disease—Geographical Distribution of Disease—Tropics: General Physiological Effects—Tropical Death-rates—Hygiene in the Tropics—Tropical Diseases—Malaria—Yellow Fever—Dysentery—Diarrhœal Disorders—Tropical Abscess of the Liver—Cholera—Plague—Sunstroke and Related Conditions—Dengue—Beri-beri—Other Minor Diseases—General Conclusions: Tropics—Temperate Zones: General—Winter and Summer Diseases—Tuberculosis—Pneumonia—Diphtheria— Influenza—Bronchitis—Rheumatism—Measles and Scarlet Fever—Typhoid Fever—Whooping Cough—Cholera Infantum—Hay Fever—Polar Zones: General—Scurvy—Climate and Health: General Conclusions.

_Introduction: Some General Relations of Climate and Health._ From earliest times people have sought in atmospheric conditions an explanation of the occurrence of disease, and have often found in statistics of mortality and of weather a more or less striking parallelism. Many fairly obvious facts naturally point to some relation of cause and effect in this matter. Some diseases are found principally in the warmer climates; others seem to prefer the colder. Some are usually more active in the warmer, or the drier, months; others have shown the contrary relation. High altitudes are free from some diseases which prevail near sea-level, and have certain favourable climatic characteristics long recognised in the treatment of disease. The pure air, increased respiration, and deeper breathing are stimulating and health-giving; they are beneficial in many affectations of the lungs, although occasionally over-stimulating in nervous and cardiac troubles. In the case of other diseases, again, altitude has no effect. Dry climates, especially deserts, whose air is usually exceptionally pure and aseptic, are generally healthful, and are beneficial in many cases where mountain climates are too stimulating. The climates within forested areas have proved especially favourable in cases of phthisis. Ocean air, pure and dust-free, with its saline constituents and equability of temperature, is beneficial to most persons as a moderate tonic and as a restorative in many illnesses. Winds are important agents in promoting health. The cool, refreshing sea breeze of the tropics brings in pure air from the sea, and is one of the most important desiderata in hot climates. Winds are active ventilating and purifying agents where population is congested. Fogs and clouds, by cutting off sunlight, weaken one of the best agents in promoting health, for the germicidal action of sunlight has been proved by many investigators. Sternberg has called it “one of the most potent and one of the cheapest agents for the destruction of pathogenic bacteria,” and says “its use for this purpose is to be recommended in making practical hygienic recommendations.” In London, a higher death-rate after a long fog may, however, result from the lower temperature during the fog, and not from any direct effect of the fog itself.

_A Complex Subject._ Facts like the foregoing naturally prejudice one in favour of a causal connection between atmospheric conditions and disease. Nevertheless, such studies have often led to very contradictory conclusions. Diseases usually characteristic of one zone are known to spread widely over other zones. Diseases which usually prefer the warmer months sometimes occur in the coldest. Rules, previously determined as the result of careful investigation, often break down in a most perplexing way. Some of the difficulty in this lack of agreement results from untrustworthy statistics, often collected under very varying conditions and really not comparable. Curves are smoothed to such an extent that they can be made to show anything. Conclusions are drawn in individual cases which are neither of general application, nor do they even apply locally on any other occasion than the special one in question. Most of this disagreement comes from the fact that not only may the different weather elements themselves, temperature, moisture, wind, sunshine, and so on, each have some effect in the production of a disease, which it is impossible to determine, but so many other factors are concerned in the matter that confusion and contradiction in the conclusions reached are inevitable. Sanitation, food, water, habits, altitude, character and moisture of the soil, race, traffic, and other controls serve to complicate the problem still further. In most studies of climate and health some, or even many, of these factors have not received attention. Hence the results have usually been incomplete. Local, peculiar, and temporary conditions may play a large part in the prevalence of disease. Overcrowding under unhygienic conditions, especially indoors during cold weather, and traffic by rail, steam, caravan, or on foot, are often more important than climate. The frequent escape of mountain, of desert, and of polar peoples from epidemics is to be attributed in most cases to the smaller chance of importing disease because of little intercourse with the outside world, and of spreading it, when imported, because of the scattered population. It may be noted, however, that the crowding indoors and the sparseness of population in these two cases are more or less directly climatically controlled.

_Climate, Micro-organisms and Disease._ The cause of disease is now no longer sought directly in meteorological conditions, but in the effects, more or less direct, of these conditions upon the micro-organisms which are the specific cause of the disease. Atmospheric conditions may help or may retard the development of the micro-organism, and may strengthen or weaken the individual’s power of resistance against the attacks of the germ, as well as affect his susceptibility. Thus new views have replaced the old. Winds used to be regarded as the chief agents in spreading epidemics: now it is known that disease cannot be carried far by winds, for the micro-organisms do not long maintain their power in the free air and under the sun. Rain has been supposed directly to control the distribution of diseases: now we believe that precipitation acts only indirectly, through drinking water, or through its control of the dust in the air. Dust from dry soil may contain the germs of infectious diseases, and aggravates affections of the respiratory organs. Harmful exhalations are no longer believed to be given off by the soil, but the condition of the soil as to moisture and temperature may affect the development and diffusion of certain micro-organisms. Some parallelism has been discovered between the prevalence of certain diseases, such as diarrhœa and typhoid fever, and soil temperatures or the ground-water level.

_Geographical Distribution of Disease._ The scheme of classifying disease geographically, on a broad climatic basis, is attractive, but not very satisfactory. For, on the one hand, many diseases are practically universal in extent, showing great independence of climate, and on the other, the history of many diseases is still in the making. In the distribution of disease too many factors are concerned to make any simple and accurate treatment possible as yet. In spite of this complexity, however, certain broad general statements may be made, useful in enabling the layman properly to co-ordinate his ideas on the subject, and fairly accurate within reasonable limits.

_Tropics: General Physiological Effects._ The uniformly high temperatures of the tropics, especially when combined with high humidity and the characteristically small diurnal variability of temperature, have certain fairly well established physiological effects. Among these the following are commonly noted: increased respiration; decreased pulse action; profuse perspiration; lessened activity of stomach and intestines, and tendency to digestive disorders; a depression of bodily and mental activity, enervation, indifference, disinclination to exertion,—in fact, a general, ill-defined condition of debility; increased activity of the liver; surexcitation of the kidneys. In damp, hot air, evaporation from lungs is slight; the blood becomes more diluted; there is a deficiency in the number of red corpuscles in consequence of the diminished proportion of oxygen in the air. There is less power to do work; greater fatigue from work; lowered vitality. All this renders the body less able to resist disease. An anæmic condition in the moist tropics is widespread.

_Tropical Death-rates._ As compared with the death-rates in colder latitudes, tropical death-rates average high. They range from the appalling rate of 483 per 1000 among European troops on the Gold Coast in 1829-1836, through 121 per 1000 for European troops in Jamaica in 1820-1836, down to so low a rate as 14.84 per 1000 for British troops in India in 1896. These death-rates, however, represent such very diverse conditions of season, climate, race, occupation, soil, mode of life, food, dwelling, etc., that they cannot legitimately be compared with one another. The prevalence of some special disease in exceptionally virulent or widespread development will raise the death-rate of any year far beyond its usual figure. Again, the presence of some insect which causes loss of crops, and the resulting lowered vitality of the people in consequence of insufficient food, may easily swell the death-rate. Nor can these tropical death-rates properly be compared with the death-rates noted under different conditions in other latitudes. (A recent attempt to compare the death-rate among American troops in the Philippines with the general death-rate in certain American cities is an excellent example of the danger of comparing two totally different things). So various and so complex are the controlling factors that critical comparative study is not worth while. Tropical death-rates are certainly high, but this fact should not be attributed solely to the dangers of the climate. Bad sanitary conditions and lack of medical attendance account for many, if not most, of the high tropical death-rates among the natives; and an irrational mode of life explains many deaths among persons coming from cooler climates. Tropical death-rates are being reduced with remarkable rapidity in all countries which are wholly or partly under white control, and especially among European troops in the tropics. This is the result of experience with tropical conditions, and of the increased precautions which are now taken in selecting and caring for the men.

_Hygiene in the Tropics._ Under the special conditions of tropical climates, the resident who comes from a cooler latitude needs to take special precautions regarding his mode of life and personal hygiene. A rational, temperate mode of life, especially the avoidance of alcoholic excess; regular exercise; non-fat-producing food; clothing suited to the climate, such as duck or linen for outside garments during the day, and light woollen for the cool of the evening and night; careful attention to the site and construction of dwellings; all possible sanitary precautions; keeping cool during the warmest hours and season by the use of fans or punkahs, by frequent baths, and by abstaining from hard work; protection against mosquitoes by means of screens; frequent change of climate by returning to cooler latitudes,—all these are important. It seems like a contradiction, but it is a fact, that the danger of taking cold in the tropics is very great, and must be carefully guarded against. General Wolseley is reported to have said of the tropics, “not to get cold is to avoid almost certainly all the causes of disease,” and a recent writer has well said that these words should be inscribed on the walls of all barracks in the tropics. The situation may be summed up in the rule: “Respect the sun, and rain, and wind; clothe with a view to avoiding chill, and live temperately.” The danger of becoming chilled is greatest during the cooler hours of evening and night, during rains, or when cool winds blow. The skin does not react well in the tropics, hence chills are frequent with even slight temperature changes, especially when there is wind. As to the best style of dwelling for the tropics, there is no absolute agreement. The material can best be determined by the local conditions in each case. Wood, stone, and thatch are employed successfully. Of whatever construction, houses should be roomy and airy, and protected against direct sunshine during the hottest hours of the day.

_Tropical Diseases._ In addition to the physiological effects just considered, certain diseases are so much at home in the tropics that they have come to be known as _tropical diseases_. This designation, however, as Sir Patrick Manson uses it in the title of his famous work, does not mean diseases confined to the tropics, but is employed in a meteorological sense for diseases associated with, but not solely or even directly due to, high temperatures. Tropical climatic conditions, _per se_, probably do not injuriously affect the natives of the tropics any more than do the conditions of extra-tropical climates affect those who live in them.

Sir Patrick Manson has made the fact very clear that the difference between the diseases of tropical and extra-tropical latitudes lies in the specific cause of these diseases. For the development of certain disease germs, certain temperatures are required. Sometimes the temperature is too high; sometimes too low. Again, certain media are necessary in propagating certain diseases, as _e. g._, a third organism, other than the disease germ itself, and man, who has the disease. The third organism may be a tropical species, as in the case of the tsetse fly; if so, the disease is a tropical disease. The opportunity for contracting the disease is best, or exists solely, in the tropics. Again, some diseases are the result of toxins generated by germs living in an external medium. One condition of development of these germs may be a certain high temperature. Thus the disease is a tropical disease, _e. g._, beri-beri. On the other hand, when everything seems favourable, natural enemies of the germs themselves, or of the organism which subtends the germs, may destroy them. Dr. Manson’s conclusion, which is the result of careful study, may well be accepted as an authoritative statement. “The more we learn about these diseases, the less important in its bearing on their geographic distribution, and as a direct pathogenic agency, becomes the _rôle_ of temperature _per se_, and the more the influence of the tropical fauna.”

Besides the more or less direct effects of exposure to tropical sun and heat, such as sunstroke, heat exhaustion, and the like, there are malaria, in varied forms, and dysentery, the two worst enemies of white residents in the tropics; dengue; ulcers; yaws; tropical abscess of the liver, a common and dreaded disease; diseases like yellow fever, cholera, and plague, which are more or less limited to certain localities, and are being hemmed in more and more by modern sanitary measures; many other infectious diseases which are common to colder as well as warmer latitudes; and beri-beri, elephantiasis, and other diseases which attack the coloured race chiefly, and are therefore of a medical rather than of a practical interest to white people. The fact that plague, and leprosy, and to some extent cholera as well, are practically limited to the tropics, is the result of modern sanitary precautions in the extra-tropics. The unsanitary conditions among tropical peoples favour the spread of these and similar diseases, and not the climate _per se_. Nevertheless it is as clear as day, in the words of Dr. Manson, that these very unsanitary conditions are “more or less an indirect outcome of tropical climate.” There is a greater variety in tropical than in extra-tropical diseases, but then many diseases common in cooler latitudes prevail also near the equator, and many diseases prevail near the equator which have practically been banished from higher latitudes. Tropical climate is not the sole, or even in many cases the determining factor. Most tropical diseases attack both natives and whites; sometimes the former suffer most; sometimes the latter. There is no rigid rule; but the racial element is often very potent.

_Malaria._ Malaria, next to tuberculosis one of the most important of diseases, was formerly considered a poisonous, gaseous emanation from the soil. It is now known to be a germ disease. In 1880, Laveran, a French army surgeon in Algiers, discovered a parasite in the blood of malarious persons. Manson later suspected mosquitoes as the means of propagating the malarial parasite. (Dr. A. F. A. King, of Washington, D. C., had advanced a similar suggestion in 1841.) Ross, at Manson’s suggestion in 1894, followed up the clue in India, and established the fact. His work, and that of Grassi, Koch, and others, has shown that the insect here concerned is a mosquito of the single genus _Anopheles_, and that malaria is due mainly, if not solely, to the injection of the parasites into the blood of human beings by the bite of mosquitoes previously infected by stinging some human being suffering from malaria.

Malaria is very widely distributed, from the polar circles to the equator, but the endemic foci, Manson points out, tend to become more numerous towards the equator. There is, on the whole, a fairly regular decrease in frequency and in severity from equator poleward. In certain parts of the tropics, as, for example, the Gold Coast, the mouths of the Congo and Zambesi, New Guinea, etc., malaria is so prevalent and so severe that the question of residence there for the white race has been practically controlled thereby. The disease is commonly associated with swamps, and moist low-lying districts, while uplands and well-drained areas are usually less affected. This relation, however, seems to be somewhat less apparent in the tropics than in higher latitudes. Malaria is perennial in the tropics, with a general tendency to a maximum in the warmer or rainy season. In the temperate zone the maximum is in late summer or early autumn.

It is clear, with the mosquito theory so well established that Koch can say, of tropical Africa, “where there are mosquitoes there is malaria, and where there are no mosquitoes there is no malaria,” that the older views regarding the relation of climate and soil to malaria must have undergone some change. Nevertheless, there is still a fairly definite relation of cause and effect in this matter. For the development of the malarial parasite in the body of the mosquito a certain degree of heat is necessary, probably a mean temperature of at least 60° F. Hirsch pointed out, some years ago, that 60° F. is the limit at which malarial fevers can occur. Hence it happens that the same mosquito may be harmless at low temperatures and dangerous at higher. Rainfall is important because the malaria-bearing mosquito passes part of its life in water. Hence lakes, and especially marshes, pools, and swamps are critical controls as breeding-places of the mosquitoes. Rain thus differs in its effects according to the amount of precipitation, and according to the conditions present where the rain falls. A rain which in one place floods and scours out mosquito-breeding pools, in another may just suffice to fill hollows and low-lying places where mosquitoes may then breed. Digging up the soil, whether for the first time or not, may result in hollows where puddles and pools may collect, and thus give rise to malaria. The ground-water level, by affecting soil moisture, also plays a part, but decomposing vegetable matter is no longer believed to be an essential. Many occurrences or non-occurrences of malaria, unexplained on any meteorological grounds, may be ascribed to the presence or absence of the malaria-bearing mosquito.

The best prevention of malaria is to screen persons who have the disease, so that they cannot infect mosquitoes, and to screen all doors and windows so that healthy individuals may not be bitten by infected mosquitoes. Wholesale protection of this kind has recently been attempted in Havana, on the Isthmus of Panama, in West Africa, and elsewhere. The danger of being bitten by the _Anopheles_, whose habits are chiefly nocturnal, is greatest at night, but residence in tropical malarial districts for white persons is always safest away from native huts and villages. The draining and filling up of swamps, pools, and puddles; levelling of the surface of the ground; cultivation of the soil by planting trees or other forms of vegetation; destruction of the larvæ by pouring oil on the standing waters; location of dwellings on high, dry sites; having these dwellings properly screened,—all these precautions should be taken. Further, a rational and scientific use of quinine, and a change of climate to a higher latitude, are both very important measures in case of the contraction of the disease. Residence at an altitude of a few thousand feet, where the temperature is lower than at sea-level, is usually a sure preventive, but the mountain climates may be injurious to persons suffering from heart or lung troubles, or from rheumatism.

Relapses are very common after a malarial attack, and an anæmic condition may continue for a long time. According to Koch, these relapsing cases infect the new mosquitoes each spring, but the same authority believes it possible to destroy all the parasites in such cases, before the spring comes, by the use of quinine.

Malaria is one of the greatest obstacles in the way of white occupation of many tropical countries. Ross spoke well when he said that the success of imperialism depends largely on success with the microscope. The hope for the future lies in the determined effort to destroy the malaria-bearing mosquitoes, and to protect individuals from infection by these mosquitoes. Preventable, to a large extent, malaria certainly is, but it is beyond the range of human power to eradicate the disease, certainly within any time which is of present political interest. In the light of the new discoveries, however, white residents in the tropics are now in far less danger from malarial infection than they were a few years ago.

_Yellow Fever._ Yellow fever is endemic only on the eastern coast of the Americas, and on the western coast of Africa, chiefly within the tropics, although it frequently extends beyond them, as an epidemic, even to latitudes between 40° and 50°. It frequents especially the squalid quarters of seacoast towns and the shores of large navigable rivers, readily following railways, canals, and other highways of travel. The opening of the Panama Canal and the establishment of new steamship lines between Central America and the Hawaiian and Samoan Islands, where no yellow fever has occurred, may easily be followed by the introduction of the disease into those islands. Within the tropics the rainy season brings the maximum prevalence of the disease; in extra-tropical latitudes, the summer and autumn. Hirsch asserts that it has not gained a foothold at temperatures below 68° F. Manson states that a temperature over 75° F. is needed for its development in epidemic form. Yellow fever weakens as cold weather approaches, and epidemics disappear when the temperature reaches 32° F., although the vitality of the germ may not be extinguished by frost (Manson). Stations more than a few hundred, or thousand, feet above sea-level are free from the disease, probably because of their lower temperatures. The altitude of this zone varies, but at the maximum, yellow fever has only very rarely occurred as high as 4000 feet above sea-level.

The actual cause of yellow fever is still unknown. The brilliant work of Reed, Carroll, Agramonte, and Guiteras has shown that the intermediate host, and the diffusing agent of the yellow fever parasite is a mosquito of the genus _Stegomyia fasciata_, which has previously been infected by biting a person suffering from yellow fever. The disease is non-contagious where _S. fasciata_ is not present, as at Petropolis, near Rio de Janeiro. Vigorous campaigns against the mosquito have recently produced a remarkable decrease of the disease at Havana, on the Isthmus of Panama, and at New Orleans in 1905. The endemic character of yellow fever in Rio is believed by Manson to be kept up by the continual arrival of foreigners who are susceptible to the disease. Newcomers are chiefly attacked. After one attack, immunity is usually secured. Persons who have lived for some time in endemic areas without having the fever are more or less exempt, or may have the disease in mild form. The immunity of natives who leave their home decreases with the length of their absence. Negroes enjoy comparative immunity; the yellow race is more, and the white race most, susceptible. Of the white race, northerners are more susceptible than southerners.

_Dysentery: Diarrhœal Disorders._ Dysentery occurs epidemically in all latitudes, but has its home in the warmer climates, as a whole increasing in severity and frequency with approach to the equator. Some form of dysentery is almost always present in lower latitudes, where this disease is next in importance to malaria in causing high death-rates and in its lasting effects. High temperatures are clearly necessary for the development of the disease germ, but numerous other controls are also needed. The maximum is usually in the hottest, or wettest, months; cooler weather checks the disease. In India, the latter half of the rainy season shows the maximum. Altitude cannot be relied on to give relief from dysentery; residents on mountains often suffer more than those at lower levels. Lack of sanitary precautions, impure water, overcrowding, poor food, excesses of all kinds, are predisposing causes. The best preventive is a rational, temperate mode of life; protection of the more susceptible parts of the body against chills, and a proper regulation of the whole system. Epidemics of dysentery seem independent of the effects of wind, rain, and atmospheric humidity. Immunity is not secured after one attack, several attacks being common.

In extra-tropical latitudes, diarrhœal disorders show a similar dependence on temperature, for they are most frequent in summer and early autumn. Usually the hotter the summer, the greater the prevalence and the severity of these complaints, and the higher the death-rate from them. Other factors are, however, concerned in this matter, so that “all attempts to express the diarrhœal mortality of a given place as a function of the temperature only have failed.” Soil temperature is one factor between which and the death-rate from diarrhœal disorders some relation has been made out.

_Tropical Abscess of the Liver._ Rare in temperate and cool climates, tropical abscess of the liver, as the name implies, is mainly a disease of warmer latitudes and usually accompanies or follows dysentery. Among the predisposing causes the most potent are injudicious and intemperate habits, especially over-eating and over-indulgence in alcoholic beverages; insufficient exercise; exposure; chills, and in general the “congestive and degenerative conditions incidental to tropical life.” Heat, malaria, and dysentery are active precursors of liver abscess, in that they lower the vitality. The disease is most common during the colder or rainier season, when chills are most frequent, but temperature is not the sole control. The physiological adjustment of a person from a colder latitude to tropical conditions of climate throws a considerable strain upon the liver. The result, especially if intemperate living is indulged in, is likely to be liver abscess. Chiefly because of their disregard of proper hygiene, white men and women are generally more liable to have the disease than natives; the death-rate among white troops in the tropics is much higher than among native troops in the case of this disease. Tropical liver abscess is most, but by no means solely, to be expected in the earlier years of residence in the tropics. Persons suffering from the disease should, if possible, be sent to a temperate climate, although there are many cases of recovery even in the tropics.

_Cholera._ Cholera is due to the specific microbe, the _comma bacillus_, discovered by Koch in 1883. From its home in India, it has spread in great waves as an epidemic over most of the globe, the last advance reaching its maximum extension early in the decade 1890-1900, in northern Europe. Cholera has gone as far north as Bergen in Norway, and in Siberia up to about latitude 60° N. No general relations can be established between the occurrence of cholera and climatic or weather conditions. Local conditions exercise an important control. In higher latitudes, however, cholera seems most frequent towards autumn, decreasing with falling temperatures. Cholera is chiefly prevalent in low-lying places, on river banks, and where human beings are over-crowded under unsanitary conditions. The atmosphere is clearly not the agent for carrying the bacillus, for the latter does not keep its morbific character long in the free air. The principal agent in spreading the disease is traffic; but drinking-water certainly also plays a part. As a whole, cholera is rarer and milder in the higher latitudes, and has decreased in Europe in cold weather, coming up again in summer. It has, however, also been active at low temperatures. With many exceptions, there may be said to be a decrease with altitude, and soil moisture may also play a part.

_Plague._ The specific cause of plague is a bacillus discovered by Kitasato, a pupil of Koch, in 1894, and also independently by Yersin. Formerly very widespread, plague is now confined to the sub-tropical districts of southern Asia and of the Mediterranean. It has become a disease of warm climates, because it depends upon the unsanitary conditions in which tropical natives live, and it attacks the poorer part of the population. Filth, famine, social misery, and overcrowding are predisposing causes. The conclusions regarding the relation of plague to weather and climate are almost as numerous as are those who have investigated this subject, but it is clear that plague is not limited by isotherms, and that meteorological conditions do not spread it, or solely control it. The Indian Plague Commission concludes that there is no direct connection between plague and climate; Hirsch had previously stated that the relation is unsettled. In the tropics, however, the disease has, on the whole, had a cool season, and in higher latitudes a warm season, maximum. As to altitude, plague has occurred at high levels in cold, dry climates, and at low levels where the climate is warm and moist. It has prevailed when the temperatures were so low that people suffered with the cold (Roumelia, 1737-8), and at temperatures so high that sunstrokes occurred (Smyrna, 1735). On the whole, plague has chiefly prevailed under moderately high temperature and moisture conditions, and where the soil is damp and the ground low. These facts do not, however, necessarily point to cause and effect.

The best preventives of plague are pure air and modern sanitation. In India, Haffkine has been very successful with inoculation. Plague travels by trade routes. Persons sick with, or incubating plague, and infected clothing and personal effects, carry the infection.

_Sunstroke and Related Conditions._ Several disagreeable and some fatal results of heat and humidity, not to be classed as diseases, are common in the tropics, and to a considerable extent also in extra-tropical latitudes, even as far as latitude 50° to 60° N. Sunstroke and heat prostration are most common in the tropics when the air is damp during calms, and in temperate latitudes during the hottest spells of summer, when the weather conditions are tropical in character. The germ origin of sunstroke has been maintained by Sambon, but the cause is to be found in the effects of insolation, direct and reflected; the air temperatures, and the undue heating of the body. The skin of white persons when exposed to the sun in the tropics often becomes burned and blistered, and travellers commonly suffer because of lack of protection of neck or limbs under sunshine. Exposure to the sun does not always explain sunstroke, for at sea the tropical sun is less fatal than on land,[26] and places with apparently similar conditions of insolation differ much as regards the prevalence of sunstroke. A great deal doubtless depends on occupation. Many forms of heat exhaustion are induced by exposure to high temperatures, but greatly aggravated by unsuitable clothing, impaired physical condition, and intemperance.

[26] Stokers and firemen suffer from prostration on steamers in the tropics, but here artificial heat is partly responsible.

A study of the sunstroke weather of August, 1896, in the United States, led Dr. W. F. R. Phillips to conclude that the number of sunstrokes followed the excess of the temperature above the normal more closely than it did any other meteorological element; that there was no definite relation to the relative or absolute humidity; and that the liability to sunstroke increased in proportion as the mean temperature of any day approached the normal maximum temperature for that day.

Sunstroke is most common among those who are exposed to the sun, and at hard work under conditions which retard or check the cooling of the body by radiation or conduction. The best protection against sunstroke and heat prostration in general, and especially in the tropics, is to be found in the use of suitable light and loose clothing; loose, wide-brimmed, and well-ventilated headgear; avoidance of exposure to sun and to high temperatures in general; the use of a white umbrella; avoidance of alcohol and of an excess of heating foods, and in a temperate life in all respects. Poor health, fatigue, and violent exercise are all predisposing causes. Tropical camps should be located in cool and well-ventilated places, and tents should have double roofs.

_Dengue._ Dengue is a “highly infectious, febrile disease, characterised by severe rheumatoid pains in joints and limbs, and in some cases by a cutaneous eruption of varying character and duration.” It is distinctly a disease of warm climates, although it has occurred as far north as latitude 40° in Europe and in North America, and as far south as the southern tropic. It comes mostly in the hottest months, and is almost always checked by cold weather. Moisture has a subordinate influence. Dengue resembles yellow fever in its prevailing preference for coasts, deltas, and large river valleys; in its relation to overcrowding and unsanitary conditions, and in its advance along routes of travel. Dengue attacks any race, and immunity is not secured by one attack. There is often a recurrence.

_Beri-beri._ A dropsical affection, combined with a disturbance of motion and sensation, and of heart action, beri-beri is found principally in or near the tropics, being especially common in the Malay Peninsula, and the adjacent archipelago, where it is often a scourge. It is especially liable to break out among gangs of labourers. Beri-beri epidemics are most common during the rainy season. High temperature and dampness are controlling factors, as are poor health, fatigue, privation, chill, overcrowding, etc. Damp years are apt to be marked by the severity and prevalence of beri-beri.

_Other Tropical Diseases._ Among other tropical diseases may be named sleeping-sickness, limited to tropical Africa and attacking both native negroes and whites; and yaws, also distinctly tropical in distribution, requiring high temperature and moisture, found chiefly in some of the larger island groups, and principally affecting the negro.

_General Conclusions: Tropics._ All parts of the equatorial zone are not equally disagreeable or hostile, so far as occupation by the white race is concerned. Many elderly persons and those who are overworked may find rest from nervous tension in the enervating climate of the tropics. The drier districts are to be preferred to the moister, the higher altitudes to the lowlands, coasts and islands well ventilated by prevailing winds, to regions where the air is stagnant. Much-needed relief from the heat at sea-level may be obtained by resort to tropical mountain stations, and many of these have become well-known health resorts. Tropical mountain climates resemble the climate of the temperate zones in their lower temperatures and in certain other ways, but they can never be the equivalent of a temperate zone climate, for they lack the seasonal changes. Some tropical climatic characteristics disappear with altitude, while others change little. The non-seasonal character of tropical mountain climates, the so-called “perpetual spring,” is not by any means the best fitted for man’s physical and mental development, however pleasant it may be for a time. With increase of altitude, there is a decrease in, or a disappearance of, some of the diseases which prevail near sea-level, such as malaria, yellow fever, liver abscess, etc. When introduced from the lowlands, such diseases are not likely to be severe, or to spread. In their stead, however, may come an increasing frequency of diseases which are characteristic of high latitudes, such as rheumatism, and heart and lung troubles. Tropical hill stations in India show a smaller mortality among the troops than do lower levels. In India, as elsewhere in the tropics, hill stations are beneficial in restoring those who are exhausted by overwork or by the heat of the lowlands. They are especially advantageous for delicate women and children. Nevertheless, climates which are temperate because of altitude in the tropics cannot replace climates which are “temperate” because of latitude.

The acclimatisation of the white race in the tropics is a question of vast importance. Upon it depend the control, government, and utilisation of the tropics. It is a very complex problem, and it has been much discussed. It is complicated by the controls exercised by race, diet, occupations, habits of life, and the like. To discuss it fully is impossible in this place. The gist of the matter is this: White residents from cooler latitudes on coming into the tropics must adjust themselves physiologically to the new climatic conditions. During this adjustment there is more or less strain on various organs of the body. The strain may be too severe; then the individual suffers. The adjustment is usually much retarded and hindered by a persistence in habits of food, drink, and general mode of life which, however well suited to the home climate, do not fit tropical conditions. During the adjustment, especially if complicated by irrational habits, the body is naturally sensitive to the new diseases to which it is exposed. Even should no specific disease be contracted, there are anæmic tendencies and other degenerative changes. Experience teaches that white men cannot with impunity do hard manual labour under a tropical sun, but that they may enjoy fairly good health as overseers, or at indoor work, if they take reasonable precautions. Acclimatisation in the full sense of having white men and women living for successive generations in the tropics, and reproducing their kind without physical, mental, and moral degeneration,—_i. e._, colonisation in the true sense,—is impossible. Tropical disease and death-rates, as has been abundantly shown, can be much reduced by proper attention to sanitary laws, so that these rates may be not much, if any, higher than those in the extra-tropics. And with increasing medical knowledge of the nature and prevention of tropical diseases, as well as by means of modern sanitary methods, a white resident in the tropics will constantly become better able to withstand disease. As Manson has put it, acclimatisation is less “an unconscious adaptation of the physiology of the individual” than “an intelligent adaptation of his habits.” For greater comfort, for better health, and for greater success, properly selected hill stations will, however, always be essential to northerners who have to live in the tropics, especially to white women and children.

It has been well said that the white soldier in the tropics is “always in campaign; if not against the enemy, at least against the climate.” This sentence may be made to fit the case of the white civilian in the tropics by making it read: the white race in the tropics is always in campaign against its enemy, the climate.

_Temperate Zones: General._ Far from being temperate as regards the general climatic conditions over much of the land area of the so-called temperate zones, these belts rightly deserve their name only in the sense that in their physiological effects they are intermediate between the equatorial and the polar zones. In the temperate zones the organs of the body act more equally than in the warmer or the cooler latitudes. In the central part of the temperate zones, especially over the continents, are found the four seasons. The winter cold is met by means of warm clothing, heated houses, and other means of protection. Unless too severe, or too prolonged, when deaths by freezing may occur, the cold of a continental winter in the north temperate zone acts as a healthful stimulant upon body and mind. In the tropics, the body is unused to adjusting itself to temperature changes, because such changes are there slight, and is readily affected by them. But the frequent, sudden, and severe changes of many parts of the temperate zone are usually borne without serious discomfort or injury, if the body is in good health, and is accustomed to adjusting itself readily to these changes. The habit of keeping houses very warm during the winter, and of having the air indoors very dry, weakens the body’s power to resist the great cold outdoors, especially if the air be damp, and causes affections of throat, lungs, and nose. The summers, although hot in the lower latitudes of these zones, and marked by spells of warm weather even to their polar limits, are not characterised by such steady, uniformly moist heat as is typical of the tropics. When the heat is extreme, and the relative humidity is high, night and day, sunstroke and kindred affections are occasionally noted in places, but the invigorating cool of autumn and winter are never far off, and may always be trusted to bring relief.

_Winter and Summer Diseases._ It is natural that marked seasonal and sudden weather changes, such as those which characterise much of the temperate zones, especially in the northern hemisphere, should be reflected in the character, distribution, and frequency of the diseases which are found in these zones. Diseases of the respiratory system, bronchial and rheumatic affections, diseases that result from colds and chills, pneumonia, bronchitis, influenza, diphtheria, whooping cough, are all common in climates with sudden marked temperature changes, especially if those changes are accompanied by cold, damp winds. These diseases are also most frequent in the winter months, when the weather changes are more common and more severe, and when, in consequence, the vitality of the body is lowered and its power of resistance against the attack of disease germs is weakened. A greater prevalence of diseases of the respiratory organs, catarrhs, and rheumatic affections in cool, moist weather, with sudden changes, has been shown by Weber, and several investigators have found a higher mortality after a greater variability of temperature. Many contagious or infectious diseases, such as diphtheria, influenza, measles, and scarlet fever, for example, are also more common in the colder season, not because the lower temperatures are the direct controlling factor, but largely because the colder weather drives people indoors; houses and buildings generally are less well ventilated; more clothing is worn, less attention is paid to personal cleanliness, and there is increased opportunity for contagion, especially among the poorer classes. Obviously, these are indirect effects of meteorological conditions. Other factors, also, must be taken into consideration. Thus one reason why the natives of the farther north, where the winters are very severe, suffer less from some of the diseases which are common in warmer latitudes is not because of the lower temperatures, but because they are less exposed to contagion owing to less communication with the outside world.

In the warmer months, fevers and diseases of the digestive system, diarrhœa, malaria, typhoid fever, are prevalent. Thus there are usually two maxima of mortality: one in the colder season, when the variability of temperature is greatest, chiefly due to respiratory diseases, and another in the warmer months, largely due to infant mortality from diarrhœal disorders.

_Tuberculosis._ “A nationally self-inflicted, unnecessary, and preventable pestilence”; world-wide in extent; found in every variety of climate, and at all altitudes; causing from 10 to 15 per cent. of all deaths; the scourge of the temperate zone, tuberculosis is, on the whole, less frequent in higher latitudes, on mountains, and in arid or semi-arid districts. Climate, however, is not the controlling factor in the latter cases, but sparseness of population and infrequency of communication with the outside world. The density of population; the social and economic conditions; the occupations and habits of the people,—these are important controls. Overcrowding amid unsanitary surroundings, absence of sunlight, impure air, are predisposing causes. Weather, or other conditions which decrease the vitality, increase the susceptibility to tuberculosis. Sudden temperature changes, especially with high relative humidity at low temperatures, cause chills and lower vitality.

Consumption, it is clear, can be successfully treated where pure air, abundant sunshine, good food, and outdoor exercise are to be had. The first of these desiderata, pure air, and plenty of it, is the most important of all. It is usually found on desert, ocean, mountain, and in forest. Hence such climates are generally advantageous in the treatment of tuberculosis of various kinds. Yet climate is no longer believed to play as important a _rôle_ in the matter as was formerly assigned to it. Good hygiene has to a large extent replaced climate. A health resort where a patient can find comfortable quarters, congenial company, plenty of diversion, and where favourable climatic conditions, such as abundant sunshine, absence of disagreeable winds, dust, and sudden weather changes, encourage outdoor life, is to be recommended. The climate does not cure; it is an important help in the treatment of the disease. Some patients, especially elderly people and those suffering from nervous, cardiac, or bronchial affections, fare better at lower altitudes; but higher altitudes, with the stimulating effects, deep respiration, and active use of the lungs which they induce, often offer many climatic conditions favourable to outdoor life and hence of great benefit in the treatment of the disease. The dry, pure air and abundant sunshine of many of the well-known mountain health resorts are very favourable climatic helps. Moreover, the smaller temperature ranges of mountain and marine climates are also helpful. In many, if not in most cases, any change of climate is beneficial, but especially so if such a change is accompanied by the favourable conditions just enumerated. Ocean air, although damp, is beneficial to many patients because of its purity, its salinity, and its small temperature ranges. Hence an ocean voyage, with its relief from unsanitary or harmful occupations, may be an excellent restorative. Results obtained in the treatment of tuberculosis by climatic change vary through a wide range. The reasons for such discrepancy are to be sought in the difference in the stage of the disease treated, and in the habits, food, and mode of life of the patients.

_Pneumonia._ Pneumonia is found almost everywhere, in the tropics probably quite as commonly as in colder latitudes, and at high altitudes as well as at sea-level. A greater frequency of pneumonia generally follows cold, damp weather, with marked changes of temperature, which lower the vitality and are conducive to chills. Hence the disease is most prevalent in the colder months. Among the predisposing causes, physical weakness following other diseases is potent, as are mal-nutrition and similar debilitating agencies. Severe cold spells are likely to be followed by an increase of pneumonia, especially among elderly persons and children. Negroes who have gone to cold climates are very subject to the disease.

_Diphtheria._ Although geographically widely distributed, diphtheria is chiefly a temperate zone disease, occurring sporadically or epidemically, however, in tropics and polar latitudes. Like other infectious diseases of the temperate zone, diphtheria is most frequent in the colder months, because the conditions of life are then most favourable to contagion, and because vitality is then most lowered by the prevailing weather conditions. Diphtheria is more common at low altitudes than high.

_Influenza._ The well-known disease, “grippe,” caused by a specific organism discovered in 1892, is occasionally very serious, and is apt to be closely followed by epidemics of pneumonia and other diseases of the respiratory organs. Although very carefully studied, there is no certain evidence of any influence of weather, climate, or soil upon the disease. The last great epidemic of influenza, in 1890 and thereabout, is believed by Assmann to have been associated with dry spells and with the carriage of dust. The worst outbreaks have been in the colder season, when indoor life, less fresh air, and overcrowding would naturally help to spread the contagion. The fact that those who are suffering from influenza are often not kept indoors explains a general spread of the disease.

_Bronchitis._ Bronchitis is most common in the higher latitudes, and in the cold months, when the temperature is low and when sudden and rapid variations of temperature are frequent. Dust, blown from the dry surface of streets and the like, helps to irritate the throat and nasal passages. Relief from bronchitis may be found where the climate is warm and uniform; the air soft and balmy; where there are no irritating winds driving the dust to and fro, and where sunshine is abundant.

_Rheumatism._ Rheumatic affections are, as a whole, more common in colder than warmer, and in damper than drier climates, but may be classed under the temperate zone. Exposure to cold and wet, bringing on chills, and sudden temperature changes, especially in damp climates, while not the cause of rheumatism lowers the vitality in such a way that the specific cause may assert itself. In many cases a change of altitude makes no difference whatever; it may, in fact, aggravate the trouble.

_Measles and Scarlet Fever._ Both measles and scarlet fever are independent of weather and climate, except in so far as the colder, more inclement, months involve an unhealthier mode of life, with less attention to sanitary measures. A maximum is usually found in the colder months, when infection is most likely. Measles occurs in all climates, but usually most commonly and most severely in temperate latitudes. Scarlet fever is essentially a disease of the temperate zone. Isolation from sources of infection is more important than any climatic control in these diseases, which show very various relations to season, altitude, and race.

_Typhoid Fever._ Typhoid fever is found in almost all parts of the world. Although common in the tropics, being one of the most generally fatal diseases there, especially among recent European arrivals, it is not, according to Manson, properly classified as a tropical disease. It is very prevalent in the temperate zone, having a maximum frequency in late summer and autumn, and is certainly largely preventable by good sanitation and pure food and water. The germs of typhoid fever are killed in a few hours under direct sunshine, and their growth is slow even in diffused daylight. The well-known studies of Pettenkofer, at Munich, showed an inverse relation between the ground-water level and the prevalence of typhoid, but this appears not to be a universal relation. The view formerly held regarding a connection between temperature and humidity and typhoid epidemics has now generally been abandoned.

_Whooping Cough._ Whooping cough is more prevalent in temperate and cooler climates, where the temperature changes are marked and where the respiratory organs are most affected, and is rare and less severe in warmer latitudes. But the absence of whooping cough is doubtless often to be explained on the ground that it has not been imported, rather than on any direct climatic basis. Although commoner and more severe in the cooler months, epidemics may occur at all times, without relation to altitude. Croup, also, prevails chiefly in damp, cool weather, with sudden changes.

_Cholera Infantum._ Among the summer diseases of the temperate zone, cholera infantum occupies a very prominent place. It increases with rising, is at a maximum with maximum, and decreases with falling, temperatures. The greater and more continuous the heat, the more general is the disease. Cool spells check it immediately. It is more common in the over-crowded and overheated quarters of the city than in the country, and may be greatly checked by the use of pure milk and fresh food.

_Hay Fever._ The specific cause of hay fever has been much debated, but is generally regarded as vegetable pollen of some sort. The particular kind of pollen may differ in different cases. The irritation is naturally confined to the season of plant growth. Relief may generally be secured by seeking a higher altitude, where the cause of irritation is absent, and where the air is pure and clean. Sea voyages, also, are beneficial.

_Polar Zones: General._ The north polar summer, as has been pointed out, in spite of its drawbacks, is in some respects a pleasant and healthful season. But the polar night is monotonous, depressing, repelling. Parry said that it would be difficult to conceive of two things which are more alike than two polar winters. An everlastingly uniform snow covering, rigidity, lifelessness, silence—except for the howl of the gale or the cracking of the ice. Small wonder that man feels like an intruder. Small wonder that the polar night has sometimes unbalanced men’s minds. Extraordinarily low winter temperatures are easily borne if the air be dry and still. Nansen notes “not very cold” at a temperature of -22°, when the air was still. Another Arctic explorer, at -9°, says “it is too warm to skate.” Zero weather seems pleasantly refreshing if clear and calm. But high relative humidity and wind—even a light breeze—give the same degree of cold a penetrating feeling of chill which may be unbearable. Thus the damper air of spring and summer usually seems much colder than the drier air of winter, although the temperatures may be the same. Large temperature ranges are endured without danger in the polar winter when the air is dry. When exposed to direct insolation, the skin burns and blisters; the lips swell and crack. In severe cold the vitality of the body is lowered, the pulse slackened, and the ability to bear hardships decreased. The surface of the body cools first; the blood circulates more slowly; the surface blood-vessels contract, and the blood then becomes internally congested; the lungs and heart may be affected, and in extreme cases death results. The danger of freezing is naturally greatest in the case of the hands, feet, ears, and nose, which are most exposed and can least well be kept warm. The skin may swell, become thick and hard, and break open. The power of resistance to extreme cold depends on the physical condition, clothing, food, exercise, exposure to sunshine, dampness, wind, and other factors. The feeling of cold is increased by hunger, and rheumatic people usually suffer most.

The physiological effects of polar cold and darkness have been fully reported upon by Arctic and Antarctic explorers. The recent expeditions, on which very careful attention has been given to picking the men, as well as to their health, diet, exercise, and general hygiene, have shown much less marked effects than did the earlier expeditions. Among the effects which have often been observed are a weakening of the senses of taste and of smell, as a result of congestion and over-secretion on the mucous membranes; depression, apathy, and sleepiness, often followed by nervous excitement and even in some cases by insanity; anæmia; tendency to digestive disorders and dyspepsia; constipation or diarrhœa; greatly lessened perspiration; fading of hair and beards; change of colour of the skin to pale and yellow; a lowering of the body temperature. The monotony of the polar night is depressing to a degree. Thirst has been one of the greatest plagues of Arctic explorers in the past. The result of a large evaporation from the lungs into the dry, cold air, thirst is a characteristic of the deserts of snow and ice as it is of the deserts of sand. The relief of this thirst by eating snow is dangerous, for it leads to inflammation of the throat and to digestive and bowel troubles. Moreover, such relief is but temporary.

It has been pointed out by physicians that, like the polar animals, the Eskimos can withstand long periods without food; that their intestinal capacity is increased in such a way that they can assimilate a constant meat diet, and that they are protected against the cold by thick, fatty tissues and by their profuse peripheral circulation.

Life is hard in the polar zones. Deaths by drowning in gales at sea, by freezing, and in snowstorms are frequent. Yet, on the other hand, most of the diseases which have been discussed in this chapter are rare or absent in the far north. There is a remarkable infrequency of infectious diseases. Polar air is very free from micro-organisms—a fact which is due chiefly to lack of communication with other parts of the world; colds are reported as rare or unknown, although changes of temperature are often frequent and large. The summer sun, both by direct and by reflected radiation, burns and bronzes the skin, and may cause snow-blindness. An Arctic summer, with its long days, crisp, clean air, and sunshine, offers conditions which are doubtless excellent for many nervous and gastric troubles, and one may predict a considerable development of summer resorts within the Arctic circle for the pleasure-loving, wealthy, and unoccupied persons of the north temperate zone.

_Scurvy._ Scurvy has been considered a polar disease _par excellence_; because it has, in the past, been prevalent on Arctic expeditions, and is found to-day in northern latitudes. Scurvy is, however, known also in many other parts of the world. It is found under conditions of overcrowding, and of poor ventilation, which are natural consequences of extreme cold. Cold is not the cause of the disease, for scurvy is found in warm countries also; but rigorous climatic conditions, poor food—especially the lack of fresh vegetables, over-exertion, depression, lowered vitality by exposure to cold, etc., are predisposing causes. The best preventives of scurvy are good food, an active outdoor life, and mental stimulation. With these precautions and good hygiene, scurvy has almost disappeared among civilised nations.

_Climate and Health: General Conclusions._ The old view concerning the paramount influence of climate upon health is being replaced by the view that good hygiene is of more importance than climate alone. Medical science has done much to stamp out some diseases like small-pox, and it will in time probably largely stamp out others, like malaria, or yellow fever, or even tuberculosis and diphtheria. Man himself, not climate, is being held responsible for the occurrence of this or that disease or epidemic, for its distribution, and for the death-rates resulting from it. Man has lowered the death-rate from disease most wonderfully. He can lower it still further. Vaccination for small-pox; preventive inoculation for plague; antitoxin for diphtheria; good food, pure air, and exercise for scurvy; draining swamps and pools and the use of mosquito netting for malaria; pure water for cholera, typhoid fever, and dysentery—these are but a few of the methods now employed by man in his war against disease.

The influence of climate is by no means to be discarded as of no account, for that it acts, in many ways, both directly and indirectly, has been shown in this chapter. The newer view regarding the influence of a change of climate as a preventive, or restorative, is that a change of residence, habits, occupations, food, is usually of more importance than the change in atmospheric conditions. If pure air, good food, freedom from worry, time for rest, proper exercise, outdoor life, and a congenial occupation are provided, many bodily and mental ailments will yield to the treatment. Climate is to be considered, because it affects our bodily comfort; it may be dull, rainy, and cheerless, or bright, sunny, and exhilarating; it may tend to keep us indoors, or it may tempt us to go out. Thus some climates will naturally be avoided, and others sought out, and the choice of a suitable climate will depend upon the disease to be dealt with. As a recent writer has well said, climate may “play an important part in the curative process, but the climate of certain localities does not possess any peculiar properties which act as a specific on certain diseases.”