44 



INFECTION AND IMMUNITY. 



animals was found suffering from the disease, and, 

 since the use of goat's milk has been prohibited, 

 the incidence of the disease in man has undergone 

 an astonishing decrease. It seems to have been 

 demonstrated that both horses and cattle occa- 

 sionally suffer from generalized infections with the 

 paratyphoid bacillus, and with Bacillus enteritidis, 

 and epidemic infections with the former have been 

 traced to the consumption of diseased meats. Bot- 

 ulism and trichiniasis are derived in the same 

 way. In some instances infection of the meat 

 probably takes place during or subsequent to 

 slaughtering. 



By contact. Anthrax, glanders and actinomycosis are con- 

 veyed to man from animals by contact, the first 

 two being contagious ; and hydrophobia by the bites 

 of rabid animals i. e., by wound infection. 



By insects. In a few instances infections are transmitted 

 from an animal to man by means of insects. Thus, 

 the conveyance of plague from the rat by the flea 

 is one of the means by which man contracts this 

 disease. The tsetse fly of South Africa, which 

 inoculates man with the trypanosome of sleeping 

 sickness, seems to derive its infection from wild 

 animals in some instances, although human pa- 

 tients are also an important source of infection for 

 fresh flies. The virus of Rocky Mountain spotted 

 fever probably passes at least a part of its exist- 

 ence in the body of one or more species of small 

 wild animals, and this step seems necessary 

 for the maintenance of virulence. In connection 

 with malaria, relapsing fever, South African tick 

 fever and the piroplasmoses a third host seems to 

 play no role, or, at any rate, not a necessary role. 



