72 INFECTION AND IMMUNITY 



Aside from the sources mentioned, the possibil- 

 ity also exists, in relation to some infections, that 

 the viruses are native to the insects, or, rather, 

 that they are habitual parasites in them, just as 

 the colon bacillus is a constant inhabitant of the 

 intestinal tract of man. Even in this case, how- 

 ever, we must assume either some extraneous 

 source for the organisms or that they are inherited 

 from the preceding generation of insects. The lat- 

 ter, indeed, is a method of acquisition which has 

 been shown to occur in ticks, in relation to the 

 Texas fever of cattle, and the South African tick 

 fever, and Eocky Mountain spotted fever of man. 

 The eggs of infected females contain the respective 

 micro-organisms and the larvae which hatch from 

 them are infective. 



Factors in The factors which enable an insect-borne disease 

 n " C to be maintained from year to year vary a good 

 deal in different cases. Chronicity, in either the 

 animal or insect host, or in both; inheritance of 

 the disease in the insect, and a rapid alternation 

 of the infection between the insect on the one hand 

 and the animal host on the other these seem to 

 be the important conditions which have a bearing 

 on maintenance in one disease or another, and all 

 of them assure a more or less constant source for 

 the fresh infection of the carriers. 



Texas fever of cattle and other piroplasmatic 

 infections, malaria, and sleeping-sickness are 

 chronic infections in both the animal and insect 

 hosts. Each exists as a more or less protracted 

 source of infection for the other. In addition, 

 Texas fever, and some other piroplasmoses, are 

 hereditary infections in the tick-carriers, and in 

 this way infection is readily kept alive from one 



