sleeping sickness is limited is a narrow region in which the climate and environment are suitable to the life history of one particular tsetse fly. Much light has been thrown on the epidemiology of plague by the discovery that it was carried to man from the rat by means of the flea. Humidity is necessary for the growth of the flea, and consequently epidemics of plague can hardly occur at seasons of the year when it is warm and dry. Thus the epidemics of plague in Bombay which have advanced progressively later and later into the year now occur when the flea is no longer at its greatest prevalence. With this change the number of cases and deaths has greatly diminished.
The epidemics of summer diarrhoea are also obviously climatic. The organism which causes the epidemic has not yet been dis- covered, but there is definite evidence that the amount of the disease is very closely associated with the summer temperature. When in London the weekly average of the air temperature rises above 60 F. and remains above that limit a large mortality is the result. Some evidence exists associating the occurrence of the disease with the presence of the house-fly, the fly carrying pu- trefying organisms from the garbage on which it feeds; but the presence of the fly and of diarrhoea at the same time does not prove that they are cause and effect. Both may well be abundant purely as, or the result of, a coincidence, the climatic conditions favouring both in an almost equal measure. A more difficult problem is the relation of weather to such infective diseases as the pneumonia of childhood. This disease is clearly associated with the winter season of the year but it does not seem specially affected by any special class of weather in that season. In the present state of knowledge it is in those diseases which depend on the spread of the organism by means of parasites that the most close association with weather has been made out.
Effect of Organisms. We now come to the question on the relation of epidemics to the organism which causes them. Why an organism should be capable at one time of causing a great epidemic and at another only a few sporadic cases of a disease has not yet been found out. That organisms do vary in the power of infecting in this manmr is a truism to anyone who has ad- ministered in the health departments of a large city. At one time the merest contact with a case of smallpox, for instance, will give rise to a large number of cases. At another time a pa- tient suffering from smallpox may even attend in the gallery of a theatre without giving rise to a case of infection.
In recent years, a considerable amount of evidence has accumu- lated that an organism having found a suitable host or succession of hosts may have its virulence unusually exalted, and if the virulence can be exalted in this manner it is probable that some similar conditions may give rise to a great increase in the power of infection. At any rate, there is no doubt that in certain con- ditions organisms become highly infective and even the best sanitary precautions exercised in such circumstances can do little more than limit the amount of the disease. But there are further considerations which arise. It would seem as if at times two series of epidemics may coincide and may even mutually in- fluence one another so as to produce a profound joint effect. Thus the great epidemic of influenza in the autumn of 1918 was associ- ated with great activity of other pneumonia-producing organisms, the result being that the death-rate was of extreme amount and was distributed with age in a manner not found in any recent epidemic of influenza.
Environment. While an epidemic may in many cases be chiefly or even wholly due to the active condition of the causal organism it is to be remembered that the vitality and environ- ment of the persons affected must also play a part. Thus, for instance, typhus fever introduced into a crowded slum in which lice are plentiful will almost certainly cause considerable havoc, but even here the havoc will be determined to a certain extent by the season of the year. If the weather be cold the people are crowded together on account of the demand for warmth, and the chance of infection is increased. In addition, in the winter food is often scarce and consequently vitality is low. If on the other hand the invasion of the organism takes place during the summer a large epidemic will be unlikely. But though these
factors act, yet if an organism has an exalted state of activity, an epidemic of the disease may occur at any season of the year, even the most unlikely. Plague, for instance, especially in tem- perate climates, is essentially a disease of the warmer part of the year, yet it has been known occasionally to occur in large epi- demics in the middle of winter, while epidemics of typhus of considerable size have been recorded in the summer time. The great epidemic of influenza in the autumn of 1918 is a marked example, such a season being in the extreme degree a very un- usual one for an outbreak of this disease. What part special susceptibility on the part of the population, due to change in vitality, played in this case is not known. Some other influences also act. There is some evidence that fatigue predisposes to enteric fever, an army on the march drinking polluted water tending to have a larger number stricken than a similarly con- ditioned civil population. Further, it cannot be doubted that the accumulated effect of seasons may tend to depress health and increase susceptibility to certain diseases. The cumulative effect of winter cold may be perhaps traced in children in re- lation to death from whooping-cough, the average minimum temperature in the winter preceding the maximum number of deaths from whooping-cough by about six weeks, while the form of the two curves is very much the same. The deaths from whooping-cough are due very largely to broncho-pneumonia, yet the seasonal distribution of whooping-cough is not identical with that of the latter disease. Thus scarlet fever, being an autumnal disease and following the hot summer, might in the same way be ascribed to depression produced by continued hot weather, making certain persons more susceptible to the disease. But as scarlet fever is a disease almost absent in warm climates this explanation can hardly be complete, and some other factor must be necessary. None of these questions, however, have at present been sufficiently investigated to allow any dogmatism. Another point of importance requires special reference, and that is the problem of " carriers," as individuals infected with a disease and cured as regards themselves, but who yet continue to harbour and distribute the parasite, are called. Cholera follows the pilgrims' way, enteric fever the carrier cook, diph- theria the carrier school-teacher.
References. The most important of the epidemiological writings of Hippocrates are the Epidemics (Books I and 3) and the treatise on Airs, Waters and Places, both included in the Sydenham Society's translation (by Francis Adams) and in Littre's text (with French translation). Galen's most important works are De Febrium Differentiis and his comments on the Hippocratic Epidemics (both in Kuehn's edition with Latin translation). The best edition of Sydenham is that edited for the Sydenham Society by Greenhill. An excellent general account of the progress of knowledge is con- tained in Haeser's Lehrbuch der Geschichte der Medizin und der epidemischen Krankheiten, 3 vols. 3d ed. (1882). English epidemi- ological history is fully related in Dr. Charles Creighton's History of Epidemics in Britain, 2 vols. (1894).
Two papers by Greenwood on the " Epidemiology of Plague in India," Journal of Hygiene, vol. x. p. 349 and vol. xi. p. 62, give examples of modern epidemiological method, while his Report " On the Rise, Spread, etc., of Epidemic Diseases," Internal. Congress of Medicine, Sec. xviii., London 1913, gives a full study with literature. Two papers by John Brownlee discussing " Theory of Epidemiology in Relation to Plague " (Proc. Roy. Soc. Med. 1918, vol. xi., p. 86) and the " Periodicities of Epidemics of Measles " (Proc. Roy. Soc. Med. 1919, vol. xii., p. 77) give an account of the statistical and mathematical methods which may be used. Ross's Prevention of Malaria and Boyce's Yellow Fever and its Prevention discuss theory and practice in all their forms. (J. BRO. ; M. G.*)
EPSTEIN, JACOB (1880- ), Anglo-Russian sculptor, was born at New York Nov. 10 1880, of Russian- Polish parents. He was educated in Paris and settled in England in 1904. He first came prominently into public notice in 1907, when he received a commission for executing 18 figures to decorate the new buildings of the British Medical Association in the Strand. His work was violently attacked, and led to a prolonged newspaper controversy, and in 1909 he produced designs for the tomb of Oscar Wilde at Pere Lachaise, Paris, which considerably shocked French taste. His other work includes the decoration of Church Square, Pretoria, and a number of portrait busts, amongst others those of Lady Gregory and Miss Iris Tree.