202 INFECTION AND IMMUNITY 



and again the Eastern Hemisphere was visited in 

 1847-48. In the winter of 1851 the disease obtained 

 wide prevalence both in North and South America. 

 In 1857-58 it again prevailed extensively in both 

 hemispheres and in some localities (Rome and Naples) 

 was attended with a high rate of mortality. During 

 the winter of 1874-75 it again prevailed extensively 

 in Europe and America. During the epidemic of 

 1889-90 the disease obtained, within a brief period, a 

 wider extension in all parts of the world than in any 

 previous epidemic. This pandemic apparently had 

 its origin in Central Asia. The United States has 

 not been free from the disease since the date last 

 mentioned, and some very susceptible individuals, es- 

 pecially in cities, suffer an attack almost annually. 



The incubation period is comparatively brief, being 

 from two to six days. Those whose occupation keeps 

 them out-of-doors during the daytime are less sub- 

 ject to attack than factory hands or others, who are 

 more exposed to contagion by reason of their liv- 

 ing in closed rooms containing numerous occupants. 

 The disease prevails to some extent throughout the 

 year but the largest number of cases occur during the 

 winter and spring months. Catarrhal conditions of 

 the bronchial and nasal mucous membranes, and the 

 meteorological conditions which are favourable to 

 "catching cold" constitute predisposing causes. Per- 



