Popular Science Monthly/Volume 52/January 1898/The Aetiology and Geographic Distribution of Infectious Diseases

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JANUARY, 1898.



IN a recent address before a medical audience I defined the term "infectious" as follows:

"It is hardly necessary to say that by 'infectious diseases' we mean those diseases which result from the introduction into the body of some disease-producing agent. And I think we are justified in saying that an essential condition of infection is that the disease producing agent shall be capable of reproduction in the body of the infected individual—in other words, that it is a living organism. It matters not whether this living organism is large or small; whether it belongs to the animal or vegetable kingdom; whether it is located in the skin as in scabies, in the muscles as in trichinosis, in the lymphatics as in erysipelas, in the solid viscera as in amoebic abscess of the liver, in the intestine as in cholera, or in the blood as in relapsing fever, the introduction and multiplication of the living infectious agent constitutes infection."

The terms contagious and infectious are not synonymous. A disease is contagious when it is transmitted from the sick to the well by personal communication or contact, more or less intimate; and all contagious diseases are infectious—i. e., they are due to the introduction into the body of a susceptible individual of a living germ. But all infectious diseases are not contagious. Thus smallpox, scarlet fever, measles, diphtheria, influenza, etc., are infectious diseases which are contagious; while malarial fevers, typhoid fever, yellow fever, cholera, pneumonia, peritonitis, etc., are infectious diseases which are not contagious—at least, they are only contagious under very exceptional circumstances, and those in close communication with the sick as nurses, etc., do not contract these diseases as a result of such close association or contact.

The generalization that all infectious diseases are due to the introduction into the bodies of susceptible individuals of living germs capable of reproduction is based upon exact knowledge, gained chiefly during the past twenty years, as regards the specific infectious agents or germs of a considerable number of the diseases of this class. In some infectious diseases, however, no such positive demonstration has yet been made.

The investigations which have been made justify the statement that each infectious disease is due to a specific—i. e., distinct—micro-organism. There are, however, certain infectious diseases which physicians formerly supposed to be distinct, and to which specific names are given which are now known to be due to one and the same infectious agent or germ. Thus puerperal fever and erysipelas are now recognized as being caused by the same germ, the germ which is the usual cause of pneumonia is also the cause of a considerable proportion of the cases of cerebro-spinal meningitis, etc.

In considering the geographic distribution of infectious diseases we will find it necessary to divide them into two groups, one in which the specific infectious agent or germ multiplies only within the bodies of infected individuals, the other in which it also multiplies external to the bodies of infected individuals when conditions as to temperature, moisture, and organic pabulum are favorable for such external multiplication.

In the first group we have all those diseases which are transmitted only by personal contagion, direct or indirect—i. e., by contact with the sick or with articles infected by such contact (fomites). This list includes smallpox, chicken pox, measles, scarlet fever, mumps, whooping-cough, influenza, and diphtheria.

In the second group we have cholera, typhoid fever, yellow fever, and the malarial fevers.

It is evident that the geographic distribution of diseases of the first group will depend chiefly upon conditions relating to the susceptibility of different races of mankind, their knowledge of preventive measures, such as disinfection, vaccination, and isolation of the sick, their mode of life and intercourse with each other and with peoples occupying different geographic areas, etc. Nomadic savages, or people living upon islands remote from the channels of commerce, are less liable to suffer from infectious diseases of foreign origin than are the denizens of populous regions, and especially of cities having commercial relations with all parts of the world. But when an exotic pestilential disease is first introduced among people who have previously enjoyed an immunity from it, on account of their isolation, it is usually very fatal, owing to the great susceptibility of a virgin population. This is due to the fact that there is no individual immunity resulting from a previous attack, and also to a relatively great race susceptibility as compared with a people among whom the disease has prevailed for many years. It is evident that the continued prevalence of an infectious disease in a given area will have a tendency to reduce the susceptibility of the population, in accordance with the laws of natural selection and survival of the fittest.

In illustration of this I may mention the comparative immunity of the African race to malarial fevers, which are so fatal to Europeans who visit the malarious regions of the African coast and interior; and the immunity of the native ("creole") population of those cities where yellow fever prevails as an endemic disease, as at Havana, Vera Cruz, and Rio de Janeiro.

What has been said will suffice to show that the geographic distribution of infectious diseases is to some extent influenced by the relative susceptibility of the population in various regions. The prevalence of the strictly contagious diseases also depends to some extent upon climatic conditions, although to a far less degree than is the case in our second group, which includes diseases in which the germ may multiply external to and independently of infected individuals.

In general, contagious diseases are more likely to spread in northern latitudes, and during the winter season, because the climatic conditions lead to the aggregation of individuals in towns and in closed apartments, while in southern latitudes and during the summer season a larger proportion of the population live in the open air during the daytime and sleep in well-ventilated rooms at night.

The influence of season upon the prevalence of smallpox, a strictly contagious disease, has been referred to by numerous authors, and is insisted upon by Hirsch in his Handbook of Geographical and Historical Pathology. In a table contained in the monumental work of Hirsch the season is given in which ninety-nine epidemics of smallpox reached their height. In sixty-seven it was during the cold season and in thirty-two during the warm season. The same thing is shown by the mortuary statistics of various civilized countries. The immunity resulting from vaccination has largely influenced the geographic distribution of smallpox epidemics, which are now almost unknown in Germany and are comparatively infrequent in the United States, in England, and in other countries where the value of vaccination is pretty generally recognized.

The influence of climate, and therefore of geographic distribution, upon the prevalence of certain diseases is due to its effect in increasing individual susceptibility to infection. Thus the susceptibility to influenza, to diphtheria, and to pneumonia is increased by exposure, leading to a sudden refrigeration of the body. These diseases are for this reason most prevalent in northern latitudes and during the seasons when by reason of exposure to sudden changes in the temperature there is the greatest liability to "catch cold."

It will be seen from what has already been said that the ætiology of infectious diseases does not depend alone upon exposure to infection—i. e., upon the presence of the specific infectious agent or germ, which is, however, an essential factor—but that the development of an attack may depend upon other factors which we may include under the general heads of (a) predisposing causes and (b) exciting causes. Predisposition may be either inherited or acquired. Thus the African race is especially liable to contract smallpox in its most virulent form, and the fair-skinned races of northern Europe are especially subject to fatal attacks of yellow fever. Again, certain families have a hereditary predisposition to pulmonary consumption, while others are especially liable to repeated attacks of smallpox in the same individual, etc. Youth constitutes a predisposition to certain diseases, the liability to attack being greatly diminished for scarlet fever and whooping-cough after adolescence and for tuberculosis after forty years of age. An acquired predisposition may be due to starvation or an inadequate diet as regards certain essential elements, to excessive fatigue or nervous exhaustion from any cause, to loss of blood, to alcoholic excesses, to insanitary surroundings, and in short to any of the causes which lower the vital resisting power of the individual. When such causes are general in their operation, or in times of famine, epidemics are likely to prevail, and the geographic range of these epidemics will coincide with the area in which the predisposing cause is effective.

As instances of the development of an attack from the direct action of an exciting cause (b), the specific germ being present, we may mention the effect of a recent debauch in causing an attack of yellow fever, of exposure to cold as the immediate cause of an attack of pneumonia or of influenza, of an attack of indigestion in developing a case of Asiatic cholera, of an injury to a joint as the exciting cause of a tubercular joint disease, etc.

What has already been said will show that the question of the geographic distribution of infectious diseases could hardly have been considered independently of questions relating to the ætiology or causation of these diseases. This will become still more apparent when we come to speak of the geographic range of infectious diseases in which there is an external development of the specific infectious agent, for such development is strictly limited by conditions relating to climate, soil, elevation above the sea level, etc. Thus yellow fever, cholera, and the malarial fevers are essentially diseases of warm countries, or of the summer season in those portions of the temperate zone in which they prevail.

Having thus called attention in a general way to the factors which influence the geographic distribution of infectious diseases, I shall now ask your attention to a brief account of some of the more important of these diseases considered separately, and in doing so it will be necessary to refer also to their geographic distribution in past times, or, in other words, to the history of epidemics.

Epidemic influenza, or as the French call it la grippe, is a disease which has frequently prevailed in all parts of the civilized world, and can not be said to have any definite geographic habitat. In this regard it corresponds with smallpox and other contagious diseases, but it is only during recent years that the fact of its transmission by personal contagion has been generally recognized by physicians, and indeed it is still denied by some. This fact, however, I consider to be well established. While references to this disease are found at a much earlier period, it was not until the year 1173 that it was described with sufficient accuracy by medical writers to justify the epidemic of that year in Italy, Germany, and England to be included in a tabular list of epidemics given by Hirsch. From that time to the present very numerous epidemics have occurred. Some of these have been limited to the eastern hemisphere, or to a restricted portion of it, while others have extended to the western hemisphere and have gained a wide prevalence on this side of the Atlantic, notably so the recent prolonged epidemic which dates from 1889. If we look at a list of the recorded epidemics during the present century we shall find that the disease has probably never been entirely absent from some portion of the eastern hemisphere, although it has been comparatively restricted in its range at times, and has again gained a wide extension in Europe and Asia, and has on numerous occasions crossed the Atlantic and invaded the western hemisphere. This occurred in 1807, 1815, 1824, 1830, 1832, 1843, 1848-'51, 1857, and in 1873-'75.

Bubonic plague is a fatal infectious disease which prevails at the present day in certain portions of China and other Oriental countries, and which in the past has prevailed as a devastating pestilence in Asia and Europe. Recent researches by the Japanese bacteriologist Kitasato and by the French bacteriologist Yersin have demonstrated the fact that the bubonic plague is due to a bacillus. No doubt the present limited geographic range of this pestilential disease is due to the great sanitary improvements which have occurred in European countries during the past two centuries. The experiments of Yersin show that rats become infected and die when they are fed upon portions of the body of victims of the plague. He also demonstrated the presence of the plague bacilli in dead rats found in the houses and streets of Hong Kong. This may account for the perpetuation of the disease in a country where rats abound, and where the victims of the plague are no doubt frequently exposed to the attacks of these voracious animals. The epidemics of plague which have occurred in Europe, so far as we are able to trace them, appear to have had their origin in the Orient. The French commissioners who were sent to Egypt in 1828 to study plague arrived at a conclusion which is in consonance with our suggestion that rats may play an important part in perpetuating the malady. Their researches convinced them that plague was unknown in Egypt previous to the year 543 (a. d.), and that its first appearance corresponds with the time when the Egyptians discontinued the practice of embalming the dead, and resorted to burial in the earth, which among the poorer classes is commonly done in a manner so inadequate that the atmosphere around a graveyard is usually filled with the products of cadaveric decomposition.

The pestilential disease which prevailed so extensively in Europe during the middle ages, and which was known everywhere as the black death, caused an enormous loss of life. This disease is now believed by epidemiologists to be identical with the bubonic plague of the Orient. No doubt, however, other pestilential maladies, and especially typhus, or "spotted fever," were confounded with the prevailing epidemic disease. The last-mentioned disease is sometimes known as "famine fever," on account of its liability to prevail in epidemic form during periods of scarcity of food. Typhus was not recognized by physicians as a distinct disease until about the end of the fifteenth century, and typhoid fever, which prevails as an endemic disease in all parts of the civilized world, was not differentiated from typhus until the early part of the present century. There is, therefore, considerable confusion as regards the real nature of the disease in many of the epidemics which occurred in Europe during the middle ages, and even as late as the last century. But there can be no doubt that bubonic plague was one of the chief causes of mortality. It continued to prevail in various parts of Europe during the sixteenth century, and during two thirds of the seventeenth; but during the latter part of the seventeenth century it became more and more rare, and after the middle of the eighteenth century its only permanent habitat in Europe appears to have been a limited area in the southeastern portion, from which it occasionally spread northward, without, however, extending much beyond the limits of the Balkan peninsula. During the early part of the present century it still occurred to some extent in this region, where it prevailed as an epidemic for the last time in 1841.

Typhus fever, like smallpox, is a disease which is transmitted by personal contagion, and its dissemination depends upon human intercourse. It prevails chiefly in temperate or cold regions, and is unknown in the tropics except at considerable elevations above the sea level. In temperate regions its season of greatest prevalence is the winter and spring. There is no reason to suppose that the specific germ, which has not yet been demonstrated, is able to multiply external to the bodies of infected individuals, and, consequently, conditions relating to soil, moisture, temperature, and organic decomposition are apparently without influence in the development of the disease, except in so far as they affect the predisposition of those exposed to infection. Insanitary surroundings no doubt constitute a predisposing cause by lowering the vital resisting power of those exposed to such influences. But of all the predisposing causes war and famine are shown by the history of past epidemics to have been the most potent.

The earliest reliable accounts of epidemics of this disease date from the eleventh century, but it was not until the sixteenth century that well-recorded accounts of the epidemic prevalence of the disease were made, in the first instance by Italian physicians. The disease prevailed extensively in Italy during the years 1505 to 1530. In the seventeenth century numerous fatal epidemics occurred in various parts of Europe, the disease for the most part following in the track of contending armies, and adding to the scourge of war with its devastations and the resulting scarcity of food the disastrous effects of a deadly pestilence. During the eighteenth century the disease continued to prevail in Europe, and three notable epidemics occurred in Ireland: the first in 1708 to 1710, the second from 1718 to 1721, the third from 1728 to 1731. The last two epidemics, although most destructive of life in the famine-stricken districts of Ireland, also extended to a considerable portion of England and Scotland. In 1734 to 1744 typhus prevailed extensively in eastern and central Europe; it again obtained wide prevalence in 1757 to 1775, a period of wars and famine, and during the last ten years of the eighteenth and the early part of the present century, the period of the Napoleonic wars, it again ravaged the countries over which the contesting armies passed. Ireland appears to be one of the endemic foci of this disease, and when it has invaded England or Scotland its origin has usually been traced to the "Emerald Isle," where frequent epidemics have occurred during the present century; that of 1826 to 1828 attained considerable proportions, and that of 1846 to 1847 was the most severe of the present century. The number of cases in the last-mentioned epidemic in Ireland is estimated to have been over a million, or about one in seven of the population.

Typhus has prevailed at various times in Mexico, Peru, and Chili, as a result of importation from Spain, since the year 1570, when it first appeared in Mexico. Its first introduction into the United States was at a much later date, and corresponds with the period of extensive emigration from Ireland to the United States and Canada during the present century. For the most part the disease has been confined to the emigrants themselves, or to their immediate attendants on board ship, at quarantine stations, or in the isolation hospitals to which the sick have been transferred. Although the disease has very frequently been brought to our seaport cities, it has rarely extended to the resident population of these towns, and is unknown in the interior of the country. In New York the disease spread to some extent in 1818, 1827, 1837, and 1847, and in Philadelphia a considerable epidemic occurred in 1836, and again in 1862 to 1864.

The conditions governing the epidemic prevalence of relapsing fever are very similar to those mentioned in connection with the ætiology of typhus. It is especially liable to prevail during times of scarcity of food, and indeed epidemics are very frequently coincident as to time and place with those of typhus. It is, no doubt, transmitted by personal contagion, and its prevalence is therefore largely influenced by circumstances relating to the susceptibility of individuals, their sanitary surroundings, and their aggregation in ill-ventilated apartments. It attacks more especially those individuals in infected districts who occupy the densely populated and filthy portions of towns and cities, and, as stated by Engel, is peculiarly a morbus pauperum, or disease of the poor. On the other hand, it is quite independent of climatic influences, and, so far as we know, has no definite local habitat. The specific germ of this disease was discovered by the German physician Obermeier in 1873. It is a slender spiral filament, endowed with very active movements, and is found in the blood of relapsing-fever patients during the primary febrile paroxysm and also during the subsequent relapses which are characteristic of the disease.

It is impossible to say when or where relapsing fever had its origin, but our first reliable accounts of the disease date from the early part of the eighteenth century, when it prevailed as an epidemic in Ireland and in Scotland. Subsequent epidemics have occurred in these countries on numerous occasions. The last epidemic period in the British Isles was from 1868 to 1873, when it prevailed in several of the larger cities of England as well as in Scotland. Upon the continent of Europe it has prevailed chiefly in Russia and in Germany, and the earliest reliable accounts only date back to the year 1833, when it first appeared at Odessa. In 1863 a widespread epidemic occurred in Russia, and in 1868 it prevailed extensively in Germany. It again prevailed in Germany in 1874 to 1872, and in 1878 to 1879. In North America its prevalence has been limited to a few outbreaks in seaport cities having commercial relations with infected localities in Europe. In 1844 it was brought to Philadelphia by emigrants sailing from Liverpool; in 1847 it was brought in the same way to New York and spread to some extent to neighboring towns; in 1869 it was again imported into Philadelphia, and during the two following years spread to a slight extent in this city and in the State of Pennsylvania.

If we may judge from past experience, the predisposing causes of relapsing fever are not sufficiently active in this country to give rise to a serious epidemic, even if cases of the disease should again be brought to our shores. In Egypt, in India, in China, and in the Oriental countries generally, the conditions favorable for the epidemic prevalence of this disease are more commonly met with, and there is evidence that it exists in some of these countries at the present day and has probably been endemic for a considerable period, especially in India. But it is only recently that the English physicians in India have recognized its presence, it having been confounded for many years with the widely prevalent malarial fevers of the country.

Smallpox, like typhus and relapsing fever, is transmitted by personal contagion, but the susceptibility to this disease is so general, independent of predisposing causes, that in the prevaccination period it had a wide diffusion, not only in the overcrowded tenements of the poor, but also in the dwellings of the rich and even in the palaces of kings. The writings of the distinguished Greek physician Galen, who was born about 130 a. d., indicate that he was acquainted with smallpox, but the origin of the disease is lost in the obscurity of the remote past. According to Hirsch, "the native foci of smallpox may be looked for in India and the countries of central Africa." It still prevails extensively in these countries, where vaccination is only practiced to a limited extent. In the years 1873 and 1874 the mortality from this disease in India is said to have been five hundred thousand. "On European soil the smallpox, up to the beginning of this century, or to the introduction of vaccination, had been one of the most widely distributed, most frequent, and most destructive of pestilences" (Hirsch).

The disease was introduced to the West Indies and to Mexico at an early date after the discovery of the "western world," and nearly every fresh outbreak during the sixteenth and seventeenth centuries can be traced to importation from Africa by ships engaged in the slave trade. The entire native population, not having previously been exposed to the ravages of this disease, was susceptible to infection, and "it was so disastrous that whole tribes were exterminated by it. . . . The disease reached Mexico for the first time in 1520 with troops from Spain; the number of persons swept off in a short time has been estimated at three millions and a half "(Hirsch). The disease was first introduced into one of our Atlantic seaports about the middle of the seventeenth century, when it prevailed in Boston. Since that date numerous localized epidemics of greater or less extent have occurred in various parts of the United States, but, owing to the early adoption of the practice of vaccination, it has not obtained a wide diffusion among the white population. It has, however, been very destructive to the aboriginal inhabitants of the country.

We must now turn to the second group of infectious diseases, viz., those in which the specific germ may multiply, under favorable conditions as to climate and soil, external to the bodies of infected individuals, and which have consequently a more or less well-defined geographic range.

In the case of typhoid fever and of the malarial fevers, which belong to this group, the geographic range is very extensive, while in cholera, in yellow fever, and beriberi it is more limited, as will be seen when I come to speak of these several diseases. The specific germ of typhoid fever is now well known, it having been first observed by the German physician Eberth, and independently by the celebrated German bacteriologist Koch, in the year 1880. Its causal relation to the disease was not established until some years later, but is now generally recognized by pathologists and well-informed physicians. This germ is found in the ulcerated glands of the intestine and consequently in the intestinal contents. The discharges from the bowels of typhoid patients, therefore, contain the germs of the disease, which probably multiply indefinitely if they find their way to shallow wells or streams at a season of the year favorable for such development. At all events, whether active development occurs or not, it is well established that typhoid fever is usually contracted by drinking water contaminated by the discharges of typhoid patients. To discuss the relations of this disease to season, temperature, latitude, local insanitary conditions, etc., would require far more space than is available at present. It prevails as an endemic disease in all the inhabited parts of Europe, Asia, and America, and the occurrence of epidemic outbreaks depends largely upon an unusual degree of contamination of the water supply of a community by the discharges of those sick with the disease. It may prevail at any season, but as a rule the autumn months afford more cases than occur at other seasons of the year. It is more prevalent in the temperate zone than in the tropics, but in the Orient it claims many victims in tropical regions, and especially in the densely populated portions of India.

Extended experience gained in this country and in Europe shows that the relation of this disease to local insanitary conditions is very marked, and that the typhoid mortality rate is a good index of the general hygienic conditions of a town or city, especially as regards purity of water supply and efficiency of sewage disposal.

Asiatic cholera is a fatal pestilential disease which has its permanent habitat in India, and which during the present century has repeatedly invaded the countries of Europe, and has even crossed the Atlantic and prevailed as an epidemic in certain portions of the western hemisphere. In India it has, no doubt, prevailed from a remote period, and its chief endemic seat in that country appears to be in lower Bengal. The deaths from cholera in the various provinces of India during the five years from 1871 to 1875 amounted to more than seven hundred and fifty thousand.

As regards its epidemic extension to the countries of Europe, cholera is a disease of the present century. The first great epidemic dates from the year 1817, and the disease did not disappear from European soil until 1823. A second period of prevalence in Europe lasted from 1826 to 1837, a third from 1846 to 1863, a fourth from 1865 to 1875, and the fifth and last from 1892 to the present date. The time at my disposal will not permit me to trace the origin and progress of these epidemics; but the general statement may be made that they had their origin in India, and that the progress of the disease was along routes of travel, showing that its propagation depends upon human intercourse. Since the discovery of the cholera spirillum by Koch, in 1884, the method in which the disease is spread has been established in a most satisfactory manner. We now know that the germs of the disease are found in immense numbers in the intestine of cholera patients, and even in individuals who have been exposed to infection, but who manifest no symptoms of the disease other than a slight diarrhœa. Such persons sow cholera seed with the discharges from their bowels, and under favorable conditions rapid multiplication of the germ occurs outside of the body. Infection usually occurs by the ingestion of water or food contaminated by such germs, and it has been shown with a great degree of probability that such contamination frequently results from the transportation of infectious material from the surface of the ground, from shallow pits, etc., by flies, which after visiting the most filthy places, may alight upon a beefsteak or fall into the milk jug in a well-ordered kitchen. But by far the larger number of cases result from drinking water containing the cholera germ.

The epidemic extension of cholera depends upon climatic conditions to a much greater extent than does that of typhoid fever. It is especially a disease of hot climates, and of the summer months in temperate regions. The disease may be propagated during the winter, even in cold climates, by the occurrence of a series of cases in localities especially favorable for such propagation, and in this case a recrudescence of the epidemic usually occurs during the following summer. In Russia, during the years 1853 to 1855, nearly two hundred and fifty thousand deaths occurred during the months of June, July, August, and September, and less than twenty-five thousand during the remaining months of the year. In 1832 cholera was introduced into Canada by emigrants from Ireland, and spread rapidly in the valley of the St. Lawrence. An independent importation during the same year brought it to New York and to New Orleans, from which points it obtained a tolerably wide diffusion in the United States. In 1835 it appeared for the first time in South America, on the coast of Guiana. North America was again visited by the scourge in 1848, and it continued to prevail in the United States and Mexico for several years, especially in 1849 to 1850. In the West Indies it caused a considerable mortality in the period from 1850 to 1854. During the year 1854 it again became widely prevalent in the United States. In 1865 the West Indies suffered from another severe epidemic, and in the following year the disease again established itself at three widely separated seaports in North America—Halifax, New York, and New Orleans. From the last-mentioned port it extended throughout the Mississippi Valley. During the years 1865 to 1868 the disease also committed great ravages in some of the South American countries not previously visited by it, and especially in Brazil, Paraguay, Uruguay, and the Argentine Republic. In 1873 cholera was again imported to New Orleans and spread throughout a considerable portion of the Mississippi Valley. Our exemption from an epidemic during the recent widespread prevalence of the disease in Europe is, no doubt, due to the efficient methods for its exclusion adopted at our ports of entry, and especially at New York, where several cholera-infected ships arrived during the height of the Hamburg epidemic of 1892.

Yellow fever is essentially a disease of the littoral, and especially of seaport cities in tropical and semitropical regions, but in these regions its prevalence is greatly restricted. In North America, although it has occasionally prevailed as an epidemic in every one of our seaport cities as far north as Boston, and in the Mississippi Valley as far north as St. Louis, it has not established itself as an endemic disease within the limits of the United States. In South America it has prevailed as an epidemic at all of the seaports on the Gulf, and on the Atlantic coast as far south as Montevideo and Buenos Ayres; also at several seaports of Mexico and Peru on the Pacific. At present the principal endemic foci of the disease are Havana, Vera Cruz, and Rio Janeiro. In Africa the disease is limited to the west coast, and so far as we know no epidemics have prevailed in the interior of tropical Africa, although the conditions would appear to be favorable for the development of an epidemic in case the disease should be introduced. The same is true as regards the populous regions in northern Africa and southern Asia where the rainfall is sufficient. The disease does not prevail in arid regions, or at considerable elevations above the sea level.

Yellow fever does not prevail as an endemic disease in places which have a mean winter temperature much below 65° F. (18.3° C), and as a rule epidemics are not developed at a lower temperature than 75° to 80° F. (23.8° to 26.6° C). The approach of cool weather checks the progress of an epidemic, and in those endemic foci of the disease (Havana, Rio de Janeiro, Vera Cruz) where it prevails annually it is essentially a disease of the summer months. That moisture is an essential factor is indicated by the fact that the disease does not prevail in arid regions where other conditions appear to be favorable, and that it is especially a disease of the seacoast and of the margins of great rivers. Heavy rains, however, exercise a favorable influence in checking an epidemic—probably by cleansing the streets, sewers, etc., in an infected locality. In the tropics the commencement of the rainy season often puts an end to the prevailing epidemic.

Decomposing matter of animal origin appears to form a favorable nidus for the development of the hypothetical yellow-fever germ. It is a disease of towns and cities, and especially of such as are in an insanitary condition. The writer's studies have led him to the conclusion that the infectious agent, as in cholera and in typhoid fever, is probably present in the discharges of the sick.

The early history of yellow fever is involved in obscurity, and it is doubtful whether we will ever be able to settle in a definite manner the disputed question as to its origin. Two principal theories have been advanced: one that it was endemic at certain points on the shores of the Gulf of Mexico at the time of the discovery of the New World; the other that it was imported to the West Indies from the African coast, probably by vessels engaged in the slave trade, soon after the occupation of the country by the Spaniards. It seems necessary to look for an original endemic focus of the disease elsewhere than in the West Indies, for the reason that, in the few places where it is now endemic, there is historical evidence to show that the disease was originally imported, and that prior to such importation it was unknown.

In Brazil, according to the best medical authorities in that country, yellow fever was not endemic at any of the seaport cities prior to the year 1849. From Brazilian ports the disease has occasionally been introduced to the cities at the mouth of the Bio de la Plata, and has there caused great loss of life.

According to Hinemann, yellow fever was unknown at Vera Cruz prior to the year 1699, a year in which it was widely prevalent in the West Indies. This city is now recognized as one of the endemic foci of the disease, and epidemics at other towns on the Mexican coast have usually been traced to importation from Vera Cruz. The Gulf coast of South America, and especially the Trench and English settlements in Guiana, have been frequently visited by epidemics of yellow fever. In Venezuela the disease has occasionally prevailed at Caracas and at the neighboring seaport, La Guayra. In Central America epidemics have occurred at all the principal seaports. Upon the Pacific coast of South America the disease was imported to Callao in 1854, and extended from this port to the Peruvian capital and to the principal towns on or near the seacoast. It continued to prevail to some extent until 1869.

The history of yellow fever in the United States shows that the disease is an exotic which has not found the conditions favorable for its continued development at any of our seaport cities. It is true that for many years it prevailed almost annually at New Orleans, but since efficient quarantine regulations have been enforced the disease has been excluded, and no epidemic has occurred in this city since 1878, a period of eighteen years, until the present year. The epidemics in this city attended with the largest mortality occurred in 1819 (mortality, 2,190), in 1847 (2,259), 1853 (7,970), 1854 (2,423), 1855 (2,670), 1858 (3,889), 1867 (3,093).

At Galveston, Mobile, and Pensacola on the Gulf coast, and at Charleston and Savannah on the Atlantic, epidemics were formerly of frequent occurrence, but these cities have also learned to protect themselves by suitable quarantine regulations. The last epidemic occurred in Galveston in 1867 (mortality, 1,150), in Mobile in 1878, in Pensacola in 1882, in Savannah in 1876, in Charleston in 1871.

During the latter part of the eighteenth and the early part of the present century several severe epidemics of yellow fever occurred in New York and in Philadelphia, and even as far north as Boston. In the great epidemic of 1798 the mortality in Boston was 200; in New York, 2,080; and in Philadelphia, 3,500. The last-named city suffered a series of epidemics about this time—1797 (mortality, 1,300), 1798 (mortality, 3,500), 1799 (mortality, 1,000), 1802 (mortality, 307), 1803 (mortality, 195), 1805 (mortality, 400). The immunity of these cities for many years, notwithstanding their intimate commercial relations with Havana and other infected ports, is in my opinion largely due to sanitary improvements, and especially to the construction of sewers and paving the streets; also to the enforcement of suitable quarantine regulations.

The great epidemics in the United States during the present century occurred in 1853, 1867, 1873, 1878. The epidemic of 1878 was the most disastrous known; 132 towns were invaded, and the mortality was 15,934 (number of cases about 74,000).

In Europe the ravages of yellow fever have been chiefly restricted to Spain and Portugal. This is due to the facts that meteorological conditions are there favorable for the development of the exotic micro-organism to which the disease is due, and that these countries have constant commercial intercourse with infected ports in the West Indies. The first epidemic in Spain occurred in 1700 at Cadiz. This city also suffered in 1730-'31, 1733-'34, 1764, 1780, 1800, 1804, 1810, 1819-'21. The epidemics of 1800, 1810, and 1819 were not limited to the city of Cadiz; the disease extended to the interior, and caused a considerable mortality in the provinces of Granada and Andalusia. In 1878 a limited epidemic occurred for the first time in Madrid. The first Lisbon epidemic was in 1723; the great epidemic in this city was inaugurated in 1856, and reached its acme of development the following year.

Upon the west coast of Africa yellow fever prevails principally along the coast of Sierra Leone. At St. Louis (Senegal) an epidemic occurred in 1778, the first of which we have any knowledge in this vicinity. Frequent epidemics have occurred in Senegambia, and the disease has occasionally prevailed upon the Gold coast, the Congo coast, the Cape Verd Islands, and the Canary Islands. At Nassau in the Bahama Islands yellow fever prevailed as an epidemic in 1861, 1862, 1863, and in 1869.

Another infectious malady which, like yellow fever, has a very restricted endemic prevalence is the disease known as beriberi. This prevails chiefly upon the seacoast of Oriental countries, and upon the islands in proximity to these coasts in the Pacific and Indian Oceans. It has been imported to the West Indies and to Brazil, where it prevails to a limited extent in the coast region.

The space at my disposal will not permit me to discuss the ætiology of this disease, but I may say en passant that the specific infectious agent, or germ, of the disease has not yet been demonstrated in a satisfactory manner, although claims to its discovery have been made.

My subject is too extensive to be treated in a single paper, and I am unable at present to consider many important infectious diseases of man and of the lower animals. Among these I may mention as especially important the malarial fevers, pulmonary consumption, pneumonia, leprosy, the diseases due to animal parasites of various kinds, those due to parasitic fungi other than the bacteria, contagious ophthalmia, etc. Among the most important infectious diseases of the lower animals, some of which may be transmitted to man by inoculation, are anthrax, glanders, hydrophobia, symptomatic anthrax or "black leg" of cattle, Texas fever of cattle, the surra disease of India, the tsetse-fly disease of Africa, fowl cholera, etc.


  1. From an address read before the National Geographic Society of Washington.