EPIDEMIOLOGY, the study of epidemics or the science that treats of those diseases that are known to attack a number of persons at the same time or in close succession. The es sential feature is that epidemic diseases belong to a group of infective or microparasitic mala dies which have the common property of spreading from time to time in a community. It is well known that many diseases of an epidemic character have their favorite haunts. In such places they are always present and there they are said to be endemic. It is only when they appear in large numbers of people in their fa vorite habitats that they assume epidemic pro portions. Thus there is little distinction between the two classes of disease, since the same dis ease may be at one time both endemic and epi demic. When an epidemic disease, for instance, influenza, spreads the entire world over, the word pandemic is applied. The essential feature in an epidemic disease is that it must have a definite contagium. The contagia may be either of bacterial or protozoan character. Thus cholera, dysentery, the plague are caused, as is known, by bacteria which, being carried in the ordinary paths of commerce, or by bodies of a moving population, are spread about the world. Occasionally epidemic disease is due to an ani mal parasite. This is presumed to be the case in yellow fever, and is known to be true of malaria. In malaria, as is now positively demon strated, the agent that is all-important in the spreading of the disease is one genus of mos quito, Anopheles. The parasite lives normally in the human body and is conveyed by means of the blood into the mosquito, where it under goes a special cycle of development, until it is inoculated into another human being, who de velops the disease.
The importation of epidemic and parasitic disease from Africa and Asia is now urged as a reason for extreme precaution in the contact of whites with alien races. The importation of such diseases from Africa is now considered to have contributed materially to the downfall of the Roman Empire. The negroes, according to recent investigation, are responsible for the introduction into the Southern States of ma larial fever, which they brought in their blood from Africa, and to which, themselves immune, the whites fell victims. Similarly the American negroes, according to a discovery made by Dr. Stiles in 1902, carry in their intestines the virus of hookworm with relatively slight discomfort; but not so the white population; and to this cause is set down the physical inefficiency and mental inertia which afflict so many of the Southern whites. Cholera, bubonic plague,
typhus and smallpox have all had their origin in Asia.
Further, it may be said that the essential features for the development of epidemic con ditions are (1) A virus; (2) a susceptible popu lation; (3) free intercommunication between the sick and the susceptible. In the group in which the animal parasites belong there must be (1) A virus; (2) a breeding-place outside of man; (3) a means of transport from place to place, either naturally or artificially; (4) a vehicle for the diffusion of the disease, such as the mosquito, for instance, or a contaminated water-supply, or a person suffering from the disease who travels from place to place and (5) a susceptible population. It is well known that variation in the susceptibility of populations is a very important element in the consideration of epidemics. Thus measles, whooping-cough, diphtheria and similar affections ravaged Ha waiian (formerly Sandwich) Islands with a fierceness and mortality unknown to modern times. Seasonal movements, sectional fluctua tions, time fluctuations and oscillations are in teresting features in the study of epidemics. By some writers the word epidemic is very loosely used to indicate certain waves of mental excita tion which have caused and still cause mental storms throughout the community, as the danc ing mania and similar occurrences. These can not be spoken of as epidemics in the true sense unless one wishes to use the broad term of contagion." Some of the most inter esting epidemics of modern years have been the influenza epidemic of 1890, which•traveled round the world in from three to four years, and the plague epidemic that was raging from 1895 to 1902, slowly traveling over the habitable globe. There have been no severe extensive epidemics in the United States, save the epidemic of in fluenza, for a great many years, and in civilized countries at the present time the chances for the spreading of more severe epidemics are greatly lessened by the application of the laws of modern hygiene as well as by procedures arising from the newly acquired knowledge concerning immunity. The time does not seem far distant when immunity from many forms of infectious disease may be conferred by proper serum therapy. Consult Creighton, 'The History of Epidemics in Great Britain from 664 A.D. to the Extinction of the Plague' (2 vols., Cambridge 1891-94) ; Hecker, of the Middle Ages' (trans. by Babington. 2 vols., London 1835) ; Hirsch, (The Geographical Distribution of Disease' (1893) ; Weichselbaum, (Epi demiologie) (1900) ; and (in 'Encyclopaedia Medico,' 1900).