156 INFECTION AND IMMUNITY 



of frequent occurrence in Chili and in Peru. The 

 disease has frequently been introduced into the 

 United States by immigrants from Europe, and local 

 epidemics have resulted from such importation, es- 

 pecially in New York (1818, 1827, 1837, 1847) an d in 

 Philadelphia (1827, 1835, 1847). Fortunately the 

 disease is unknown in other portions of the United 

 States, probably because our history does not include 

 any famine periods, such as have apparently consti- 

 tuted an essential factor in the epidemic prevalence 

 of the disease in European countries. Africa and 

 Asia have been to a great extent exempt from this 

 scourge, although it has prevailed in northern Africa, 

 in Persia, and in Syria. Typhus is essentially a dis- 

 ease of temperate and cold countries. In the popul- 

 ous countries and islands of the tropics it is practically 

 unknown. Epidemics occur more frequently during 

 the winter and spring months than in summer. This 

 is probably due to the fact that it is personally con- 

 tagious. Like other diseases which are communicated 

 from person to person, it is more likely to extend 

 when cold weather leads to an indoor life and more 

 intimate and continued association of individuals. 

 Again, the fact that scarcity of food is more likely to 

 occur in winter and in the early spring is probably a 

 factor of some importance in determining the seasonal 

 prevalence of the disease. 



