TRANSMISSION OF PLAGUE. 487 



sumably this is accomplished through infected rats 

 which may board a ship during its stay in a pest- 

 ridden harbor, and which subsequently escape at 

 the new port. 



Epidemics of plague lack the explosive-like sud- Epidemics. 

 denness in their development which characterizes 

 cholera and,, to a certain extent, typhoid and dys- 

 entery. The cases occur in groups and in particu- 

 lar houses in such a manner that direct and indi- 

 rect contact seem to be largely responsible for 

 transmission. Every epidemic of plague may be 

 divided into three stages: a slow progression from 

 small centers, an acme of widespread death, and a 

 slow recession (Dieudonne). It seems probable 

 that the disease spreads rapidly and extensively 

 only when the pneumonic form prevails. 



In man infection takes place through the skin JjJ tlon 

 most frequently, although the mucous membranes 

 of the mouth, nose, pharynx, tonsils or the con- 

 junctiva are possible infection atria. Often no 

 local reaction is produced, and the point of en- 

 trance may be indicated only in a general way by 

 the swollen lymph glands of the region. Infre- 

 quently a pustule or small carbuncle marks the 

 point of entrance. Primary plague pneumonia is 

 caused by the inhalation of pest-laden material, 

 particularly fine particles of sputum from a pneu- 

 monic case, and perhaps also by the inhalation of 

 infected dust ; the latter is probably of less impor- 

 tance because of the short life of the organism in 

 dust. Even in ordinary speaking minute drops of 

 saliva are thrown into the air. Infection is thought 

 not to occur through the stomach or intestines. 

 In the pneumonic and septicemic forms, the in- 

 fected urine and feces contribute to the dissemina- 



