CLINICAL AND PARASITOLOGICAL PERIODS 



a new host. These are ( i ) by ''contagion" or direct trans- 

 ference from definitive host to definitive host, (2) by 

 inoculation through the agency of an intermediate host, 

 and (3) by "inheritance." The ''hereditary" method of 

 transmission is not known to occur in any protozoon 

 Hving in man, but takes place in a number of species of 

 parasites of lower animals; inoculation by an interme- 

 diate host is a very common method of transmission 

 among blood-inhabiting protozoa. 



The direct or contagious method of transmission may 

 be brought about in various ways. Among "intestinal" 

 protozoa the amoeba, Endamocba ghigivalis (Fig. 5) and 

 the flagellate, Trichomonas buccalis (Fig. 8), that live 

 in the mouth, no doubt are transferred directly from one 

 host to another by kissing. The flagellate, Trichomonas 

 vaginalis (Fig. 7), which is apparently widespread 

 among women and has been recorded from the urinary 

 tract of man, may possibly be distributed during coitus. 



2. CLINICAL AND PARASITOLOGICAL PERIODS DURING THE 

 COURSE OF A NATURAL INFECTION 



The term natural infection is used here to designate 

 an infection in nature during which the parasite is able 

 to pass through its life-cycle successfully and provide 

 infective stages for the invasion of a new host or inter- 

 mediate host. In contrast to natural infections, are con- 

 ditions that result from the invasion of hosts that may 

 be called foreign, refractory, accidental or casual, — 

 terms that are fully explained below under the subhead- 

 ing Host-Parasite Specificity. 



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