_Sex and Age._—Both sexes are equally liable. It is said that people between the ages of 10 to 30 are frequently attacked. But experience shows that children and old people are alike liable to attack. In Bombay the disease has occurred most frequently between the ages of 20 and 30, and the male sex has suffered more than the female. _Occupation._—During one epidemic, water-carriers, or those who used much water, or who dealt in oil and fats were found comparatively free; but this observation cannot be relied upon. Those whose business requires them to come much in contact with the sick, and also those who are engaged in cleaning are naturally more exposed to infection. It has been said that one attack generally protects from a second, which, if it occurs, usually runs a mild course. _Sanitation and Personal Hygiene._—Plague, germs thrive in filth. Bad hygienic conditions, over-crowding, insufficient ventilation, and absence of sunlight in dwelling houses, accumulation of decomposing organic matter, effluvia from bad drains, sewer and cesspools are causes that favour the growth and dissemination of an epidemic of plague. When it attacks a town, it, therefore, naturally selects first the poorer classes who live in ill-ventilated and over-crowded houses. Over-crowding within dwelling houses is a fertile source of producing a constitution fitted for the reception of plague-germs. Such portions of towns where there is much congestion and over-crowding suffer most. Scarcity of food favours plague. It has been called _Miseriæ Morbus_, or the disease of misery, and the plague of London was called the “poor’s plague.” In Kumaon there is the usual custom of keeping cattle in the lower room of a hut, where a crowd of cattle stand udder deep in fœtid straw. In one of the rooms of the upper storey grain is kept, and in another the whole family sleeps with doors and windows shut. Such conditions are undoubtedly very favourable to plague. _Predisposition._—Chill and exposure to cold, indigestion and any other disease producing a debilitated condition of body, fatigue, overwork, error of diet, mental emotion, and a terror of attack are predisposing causes. Catarrhs often predispose an attack. _INCUBATION._ Or how long does it take for the poison to develop symptoms after infection: generally three to six days, but the period may be up to ten days. The period varies with the virulence of the poison. The germs, however, remain active outside a host for a long time. An instance has been reported, in which a man, after handling some ropes which 20 years previously had been used in the burial of plague corpses, took the disease and died of it. _SYMPTOMS._