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336 than forty-eight hours, and that floors of chunam cease to be infective in twenty-four hours.

Conditions favouring naturally acquired plague.—The most potent circumstances which predispose to the epidemic outbreak of plague are extreme filth and overcrowding. In such circumstances the virus, once introduced, tends to spread. These conditions, however, are not all-sufficient, for even in the filthiest and most crowded oriental towns, and without any apparent

Fig. 70.—Edge of a mass of plague culture from flea's stomach. Microphotograph x 1,000.

alteration in the habits or circumstances of the population, the disease, after having become epidemic, dies out spontaneously. It may be difficult to indicate the exact way or ways in which filth and overcrowding operate; but certain it is, as experience has shown, that in good sanitary conditions plague does not spread even if introduced, and that in opposite conditions it may for a time spread like wildfire.

Filth and overcrowding imply close proximity of the sick and the healthy; an atmosphere saturated with the emanations of the sick; a lowered tone of