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Definition.— Plague is a specific, inoculable, and otherwise communicable epidemic disease common to man and many of the lower animals. It is characterized by fever, adenitis, a rapid course, a very high mortality, and the presence of a specific bacterium, Bacillus pestis, in the lymphatic glands, viscera, and blood. In a large proportion of cases buboes form in the groins, armpits, or neck.

Geographical distribution.— Though not necessarily confined to warm climates, in modern times plague, like leprosy, has practically become so. The hygienic conditions which advancing civilization has brought in its train have forced back these two diseases from Europe, where at one time they were even more prevalent than they are in their present tropical and sub-tropical haunts. Plague and leprosy are typical examples, of that large group of acute and chronic germ diseases whose spread depends on social and hygienic rather than on climatic conditions, and more especially on the verminous accompaniments of filth and overcrowding: conditions which nowadays are found, to an extent and an intensity sufficient to ensure the endemic prevalence or epidemic extension of these diseases, for the most part only in warm countries.

It is difficult to say what the pestis of the ancients may have been. Probably in many instances it was bubonic plague; doubtless the term was sometimes applied to other epidemic sicknesses attended with a large mortality. The descriptions which have come down to us of these old-world epidemics are too vague for recognition. According to Hirsch, the first recognizable description of what is now understood by plague