EPIDEMIOLOGY. In recent years more study has been given to that branch of the science of medicine which, under the name of epidemiology, displays the general factors which operate upon populations or aggregates, and lead to the outbreak of a sickness affecting several persons within a short time. The unit of the epidemiologist is a population, of a physician an individual.
Modern epidemiology is based on the collections of statistics which began half way through the 1 gth century, and on the associated information which was ob tained as to the causation and course of epidemics by careful local enquiry into all the conditions. Even before this some countries, such as Sweden, published the figures of the deaths from numerous infectious diseases for series of years; but, though very interesting, these figures reflect more or less special condi tions. Since about 184o, especially in Europe, India and America, carefully collected information exists respecting many epidemics and epidemiological conditions. Sufficient evidence is now avail able to examine any theory which may be offered to account for the facts. Advance has been made on a number of lines : on the modes of spread of infection; on the theory of the course, recur rence and size of epidemics; on the relation of epidemics to cli matic conditions and the cause of these relations; on the knowledge of the life history of the organisms which cause epidemics; on the conditions of living Which favour the spread of infectious disease.
the discovery of the organisms (see BAC
