PARASITIC AND PATHOGENIC BACTERIA 95 



animals to man, incidental to feeding. Thus, the flea transmits the 

 plague bacillus from rat to man, from man to man, and possibly from 

 man to the rat. The louse similarly spreads the virus of typhus from 

 man to man. In the instances cited the insect is probably not a true 

 intermediary host, for the virus does not necessarily multiply in the 

 insect, nor does the virus undergo any essential transformation, so 

 far as is known, in the insect. Nevertheless, the transmission of the 

 viruses of these diseases bubonic plague and septicemia, for example, 

 depends upon the agency of suctorial insects for their passage from 

 host to host. Other insects also transmit disease, but the evidence 

 in a majority of instances is somewhat less definite than the cases 

 cited. 



(d) Intermediary Hosts. Certain insects, notably mosquitoes, 

 transmit disease from man to man only after the virus has passed 

 an extracorporeal cycle in the extrinsic host the mosquito in this 

 instance. Thus, Anopheles transmits malaria from man to man and 

 Stegomyia fasciata, or as it is now called, Aedes calopus, transmits in 

 similar manner, the virus of yellow fever. Transmission in these cases 

 is through the female insect and a definite interval (latent period) 

 must elapse between the time of biting the patient and the time when 

 the mosquito becomes infective to the non-immune host. 



6. Human Carrriers. Individuals who are apparently healthy 

 occasionally harbor within their bodies (in free communication with 

 the exterior, however, either through the respiratory tract, the gastro- 

 intestinal tract, the urinary tract or the skin) bacteria which are 

 capable of inciting disease in others. Such individuals are known as 

 bacillus carriers; frequently they eliminate these pathogenic bacteria 

 in large numbers. 



The bacillus carrier may or may not give a history indicating 

 recovery from an infection of the specific organism which he "carries." 

 Bacillus carriers may be temporary carriers, in which event they harbor 

 the pathogenic bacteria for but a few weeks; or they may become 

 habitual carriers, in which case the organism may be excreted for 

 considerable periods of time, even years. The excretion may be 

 constant or intermittent. 



The typhoid bacillus is a common organism to be thus carried. It 

 appears to localize eventually in the gall-bladder or the bile ducts, less 

 commonly in the urinary bladder, and it may appear occasionally 

 in large numbers in the feces or urine of the carrier. Women are more 

 commonly found to be typhoid carriers than men. Similarly, para- 



