As I live in a university community in which at least 20 per cent of the population leaves and returns each week-end, and in which 30 per cent of the population leaves and returns semi-annually, I have had an unusual opportunity to study several large epidemics of pityriasis rosea during the past fifteen years.
During the past five years, I have been much impressed with the marked increase in the number of cases of this disease and with the frequent occurrence of aberrant forms, many of which are extremely severe as compared with the average. Another striking feature which has impressed me is the fact that whereas the majority of cases occur during epidemics in the spring and fall, there is no time of the year in which the disease does not occur. Sporadic cases appear constantly during the summer and winter months in increasing numbers.
The evidences of direct contagion have been striking in not a few instances, and during the past four years, they have led me to undertake a series of studies with the idea of determining in what manner the disease is transmissible, and how readily it may be transmitted experimentally.