92 INFECTION AND IMMUNITY. 



The virus of hydrophobia, when passed through 

 several consecutive rabbits, and that of smallpox 

 when passed through the calf, lose in virulence for 

 man. 



of The increase of virulence which results from 

 passage has been explained by assuming that all 

 the weaker and less virulent individuals of the 

 culture are killed by the serum and leucocytes of 

 the animal, leaving the more resistant and more 

 virulent. This process of selection is probably an 

 important factor in the change, but it would hardly 

 explain the increase which occurs when an organism 

 is grown on heated serum. It seems probable, as 

 suggested by Eisenberg and others, that as a conse- 

 quence of residence in the body of the animal the 

 culture becomes immunized against the antibac- 

 terial factors (bactericidal amboceptors and opson- 

 ins) of the host, which would naturally render the 

 organism a more dangerous one for subsequent 

 animals into which it may be injected. It is even 

 possible, as suggested by Welch and others, that 

 certain elements in the body fluids may stimulate 

 the micro-organisms to a more abundant secretion 

 of toxins and other substances (amboceptors), and 

 that the increased virulence caused by passage may 

 depend on the retention of this power after leaving 

 the tissues of the host. 



It is probable that all organisms which are able 

 to live for a shorter or longer time within the tis- 

 sues of an animal cause anatomic changes and 

 abnormal symptoms sooner or later. This, of course, 

 does not apply to those which live habitually on the 

 cutaneous or mucous surfaces. Those which live 

 on the skin do not reach the interior because of 

 mechanical obstacles, and regarding those which 



