82 INFECTION AND IMMUNITY. 



in a protozoon. 11 Given a case in which the dis- 

 ease is caused by a bacterium rather than a proto- 

 zoon, we may assume, first, that the infection in 

 the insect is limited to its alimentary tract; or, 

 second, that a generalized infection of the carrier 

 takes place with a localization of the micro-organ- 

 isms in its salivary glands. In the first instance 

 a large proportion of the organisms ingested with 

 the infected blood reach the stomach and intes- 

 tines, whereas it is probable that a relatively small 

 number remain in the proboscis. When the insect 

 feeds at once, or soon, on another (healthy) ani- 

 mal the number of micro-organisms injected from 

 the proboscis may be insufficient to infect the new 

 host. Some days might be required for prolifera- 

 tion to result in an infective quantity, either in 

 the proboscis, stomach, intestines, or by contiguous 

 extension, in the salivary ducts and glands. At 

 the time of inoculation the micro-organisms might 

 simply be washed from the proboscis into the cutis 

 by means of the salivary secretion, or directly in 

 the latter, or after a certain amount of regurgita- 

 tion from the stomach had taken place into the 

 proboscis. 



In the second case, in which the insect under- 

 goes a generalized infection, the primary non-in- 

 fective period (incubation period) may well rep- 

 resent the time required for this general invasion, 

 with the consequent localization of the bacteria 

 in the salivary glands. We have a very good 

 analogy in typhoid fever in man, in which a VQTJ 

 definite incubation period precedes generalized 

 infection. 



11. The Role of Protozoa in Pathology ; Proc. of the Path. 

 Soc. of Philadelphia, 1907, pp. 1-27. 



