Plague is a very slow disease; it takes some weeks to travel from one quarter of a city to another. It took nine months to travel from the city of London to Soho, and ten from Hongkong to Macao—a distance of 30 miles. Thousands of persons from Canton and Hongkong sought shelter at Macao, and there was free communication between these places, still Macao became affected nine months after plague ceased at Hongkong. When plague is first imported in a place, for three or four weeks isolated cases occur in one neighbourhood. An epidemic may last only a few weeks or months, but may extend over several years in sporadic form and a recrudescence takes place abruptly. In Mesopotamia plague declines and becomes dormant with the setting in of the hot weather, its activity reawakening in winter and gathering force with the advancing spring. The same was the case in Egypt. In Constantinople, on the contrary, as well as in England, the disease was dormant during the cold months but became active during the hotter. In England, September was the month of greatest prevalence. In the epidemic at Bengazi in 1858 as well as in Mukai in 1863 famine and plague were found together. That a water-logged soil favours famine was illustrated during the epidemic on the Lower Euphrates in 1867. In Persia and Arabia many epidemics were self-limited and spontaneously came to an end after spreading on a certain area, while, on the other hand, it has been known to obtain an endemic foot-hold, the virus remaining from year to year, and, occasionally under the influence of meteorological or unknown causes, becoming epidemic among the population.

Race.—No race seems to enjoy an immunity from the plague.

Geology and Climate.—Except the new hemisphere the plague has found congenial soil everywhere. It thrives as much in high and dry altitudes as in low-lying places, as much in overcrowded towns as in sparsely populated semi-desert regions. In temperate regions it has been known to rage in summer, but in Astrakhan it prevailed when there were several feet of snow on the ground. On the Volga it prevailed during the severest cold (1878-79), as well as in the extreme heat of Smyrna (1735). In Bombay it commenced at the end of an exceptionally dry season, as it did in South China. During the Hongkong epidemic the rains increased it. The increase of the epidemic at Hongkong with the rains was probably due to the fact that the rains drove people into infected houses, instead of sleeping outside, as they did in summer when the weather was good. It has been said that plague flourishes in a warm moist atmosphere and dry hot air kills it, but there are records of plague thriving in conditions antagonistic to this theory. A temperature between 60° and 85°F. is said to be very favourable to it.

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 foetid 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.