CARRIERS 



since it is thus prevented from further growth and mul- 

 tipHcation and especially from dissemination. Sponta- 

 neous recovery results in most cases of protozoan infec- 

 tion, but by recovery is meant the cessation of symptoms 

 and not the total disappearance of parasites from the 

 body. Frequently the host, by means of its acquired re- 

 sistance, is able to destroy most but not all of the para- 

 sites, and hence to bring about the carrier condition. 



A carrier is a host in which parasites live and by 

 v^hich they are disseminated but which exhibits no visible 

 symptoms of infection. Walker and Sellards (1913) dis- 

 tinguished two types of carriers in their work with hu- 

 man amoebae, (i) contact carriers who are parasitized 

 but never have exhibited symptoms, and (2) convales- 

 cent carriers who have recovered from the disease but are 

 still infected. This is the ideal condition for the parasite 

 since it is not in danger of losing its host and is ensured 

 of the distribution of its offspring. As a matter of fact, 

 most host-parasite relations are of this type; hosts be- 

 come infected but never show symptoms and are appar- 

 ently none the worse because of the presence of the 

 parasite, a sort of equilibrium between host and parasite 

 being established. Certain species of hosts are almost uni- 

 versally infected in nature by certain species of parasites 

 and the parasitized condition might almost be consid- 

 ered the normal state for these species. 



Infection without symptoms is supposed to be the re- 

 sult of long periods of association. According to this 

 view the length of parasitism of a certain species of host 

 by a certain species of parasite can be determined ap- 



37 



