SPECIALIZATION IN VIRULENCE. 89 



obtain a foothold and to produce disease, even 

 when a very minute quantity has been introduced; 

 or one which., after introduction, is able to prolifer- 

 ate to an enormous degree in a particular host is 

 said to be very infective, or infectious, for this host. 



Infectivity is not synonymous, nor is it neces- 

 sarily coextensive, with virulence or pathogenicity. 

 Thus Trypanosoma lewisi for the rat, the ma- 

 larial parasites for the mosquito, and the virus of 

 Rocky Mountain spotted fever for the tick are 

 highly infective, but have a very low grade of path- 

 ogenicity for these hosts. The leprosy bacillus, the 

 spirochete of syphilis, and the tubercle bacillus 

 have marked infectivity for man, but their viru- 

 lence, as indicated by the slow progressive character 

 of the diseases, may be considered as moderate, 

 although the final event may be tragic enough. On 

 the other hand, certain organisms possess great 

 infectivity and great virulence at the same time, 

 as plague, anthrax and the glanders bacilli (acute 

 glanders) for man, and the trypanosomes of sleep- 

 ing sickness, nagana and dourine for white mice. 



The virulence of some micro-organisms is often 

 specialized with regard to particular species of ani- 

 mals. Thus measles, scarlet fever and leprosy seem 

 to attack man only, and no animal can be infected 

 with their viruses. Smallpox and syphilis are less 

 specialized. Smallpox can be inoculated into the 

 ox and to a certain degree into the rabbit, and syph- 

 ilis into monkeys, and perhaps also into the rabbit, 

 although the latter may not yet be definitely deter- 

 mined. Malaria and yellow fever infect only man 

 and the mosquitoes which carry these diseases. 

 Many other pathogenic organisms are in contrast to 

 those mentioned, in that they are able to cause 



