244 A MANUAL OF BACTERIOLOGY 



determined by races of the parasite of particular quality, 

 so that what is virulent for one species is not necessarily 

 virulent for another. Thus strains of pneumococci are 

 known of which a single organism will set up a fatal 

 septicaemia in the rabbit, but many millions of which 

 may not infect the guinea-pig. By successive transfers 

 through a susceptible animal a slightly virulent strain may 

 be rendered incredibly virulent. The changes take place 

 sometimes slowly and sometimes quickly ; in the latter 

 instance, they correspond to or suggest the appearance of 

 " sports " or " mutants " among the higher plants or 

 animals. It may well be that epidemic prevalence of a 

 disease is related to some such acquisition by the germ of 

 heightened virulence or capacity for infecting. 



On the other hand, certain parasites under particular 

 conditions acquire the power of resistance to factors 

 inimical to their existence. Under the influence of specific 

 germicidal serums and drugs, they undergo a subtle change 

 through which they acquire a capacity of effective resist- 

 ance to the germicidal agent. This state is known as 

 " fastness " and seems to be equivalent to the develop- 

 ment of sports or mutants among higher forms. Our 

 knowledge of this condition has been chiefly derived 

 from a study of trypanosomes and spirochaetes, but 

 it also occurs among the bacteria. Among the try- 

 panosomes it persists only so long as they continue to 

 multiply asexually in the blood of the host and disappears 

 when the organisms multiply sexually in the intermediate 

 host. Bacterial fastness tends to disappear when the 

 organisms are cultivated outside the body. This acquire- 

 ment of a resistant state by the parasite may be account- 

 able in some instances for the relapses which occur in the 

 course of some infective diseases, e.g. typhoid fever. 



The flushing-out action of accelerated circulation will 

 exert some action in eliminating organisms from a local- 



