Seasonal conditions may affect the course of an epidemic in various ways. (a) By effect upon flea prevalence, cold weather greatly lessening the number of insects. (b) By effect upon rats, cold weather and rains either driving them from overground to underground, or vice versa, or from their principal avenues of travel in cities (the sewers), into houses and buildings. (c) By effect upon the plague germ, _Bacillus pestis_. The resistance of this organism is very variable, sunlight and drying being its greatest enemies, while darkness and dampness are its chief allies. So far as temperature is concerned, the plague bacillus is not likely to be seriously affected by natural temperatures, as it is not destroyed by heat below 150 degrees Fahrenheit, nor by cold measured by zero Fahrenheit, which means that it survives freezing, generally speaking. It is probable that the periods of greatest seasonal prevalence of plague will be found to correspond generally with increased prevalence of rat fleas. During the periods when rat fleas are absent or least prevalent, the disease is perpetuated in the form of chronic (subacute) rat plague in a small number of the rodents. The India Plague Commission made and verified this observation. Cholera epidemics often abate spontaneously and this is believed to be due in part to attenuations of virulence and changes in the cholera organism which may be demonstrated in the laboratory. We can hardly hope for such spontaneous abatements in plague epidemics, as it has been found difficult to attenuate or to intensify cultures of plague bacilli permanently in laboratory experiments with animals. If it is true that plague epidemics are often marked by a preponderance of mild cases in the early days and a gradual subsidence of intensity of the cases as the epidemics wane, we probably will have to look to the susceptibility of our patients for our explanation of this phenomenon, rather than to variations in the virulence of the plague bacilli. If plague bacilli continue to be distributed to susceptible people the disease should continue with a general stability of virulence.