old age seems to be to a certain extent protective, the disease being rarer after 50 than during adolescence. Women, doubtless on account of their remaining much indoors in the tainted surroundings, are relatively more frequently attacked than men.
Geological constitution of soil appears to have no direct influence on plague.
Atmospheric temperatures if very high or very low seem to have a repressing effect. Thus, as a rule, epidemics in Egypt and Mesopotamia declined during the height of the very hot and dry summer, and in Europe during the extreme cold of winter. On the other hand, plague on more than one occasion has flourished during a Russian winter, and also, as in Hong Kong recently, during the heat of a tropical summer. On the whole, the evidence points to moderate temperatures— 50° to 80° F.— combined with a certain degree of dampness as being the principal atmospheric condition favouring epidemic outbreaks and recurrences. Manifestly any influence temperature may have is only an indirect one.
In large towns, and in some districts, in which plague recurs for several years in succession, there is a seasonal periodicity (which may not be the same in all places) of maximum and minimum prevalence.
Elevation, as regards sea-level, does not directly affect the general distribution of the disease. Indeed, mountain tribes, probably on account of their poverty and squalor, are peculiarly liable to epidemics. In houses the ground floor is more dangerous than are the upper storeys.
The duration of epidemics of plague is very variable. In large cities— Bombay, Hong Kong, Canton, for example— when fairly established the disease may not relax its grip for ten or more years. In smaller places it may disappear in a few months.
The extension of plague epidemics is peculiar, and in many respects resembles that of cholera. It follows trade routes. Sometimes it may spread rapidly from point to point; more generally it creeps slowly from one village to another, from one street or one house to another, Sometimes it skips a house,
