EPIDEMIOLOGY OF TRANSMISSION 



resistant walls secreted by the organism, which help pre- 

 vent loss of water; keep out injurious substances; and 

 guard against molar agents. The terms applied to 

 these resistant bodies are *'cyst" (Fig. ib) in the case 

 of intestinal amoebae, flagellates and ciliates, and "oocyst" 

 (Fig. 15) in the coccidia. 



In most cases, probably no development occurs within 

 the cysts after they escape from the body. The oocysts 

 of coccidia, however, continue their development if dis- 

 charged when still immature. Presumably not until 

 sporozoites are fully developed do the oocysts become 

 infective. 



It seems clear from the evidence available ( i ) that no 

 foci of infection of disease-producing protozoa exist out- 

 side of the host and (2) that in the majority of cases 

 certain stages are already infective when they escape 

 from the body and all other stages including trophozoites 

 and immature cysts die outside of the host. 



Plate 2 



Intestinal Flagellates (Figs. 7 to 14) and Coccidia (Figs. 15 and 17J 

 Living in Man. 



(All figures of flagellates magnified about 2000 diameters and of coccidia 

 about 1600 diameters) 



7. Trichomonas vaginalis. (After Hegner.) 



8. Trichomonas buccalis. (After Goodey and WelHngs.) 



9. Trichomonas hominis. (After Faust.) 



loa and lOb. Chilomastix mesnili. loa, trophozoite. lob, cyst. (loa, 

 after Boeck; lob, after Kofoid and Swezy.) 



iia and iib. Embadomo-nas intestinalis. iia, trophozoite. lib, cyst. 

 (After Hegner.) 



12a and 12b. Tricercomonas intestinalis. 12a, trophozoite. 12b, cyst. 

 (After Wenyon and O'Connor.) 



13a and 13b. Giardia lamblia. 13a, trophozoite. 13b, cyst. (After Simon.) 



14. Enteromonas hominis. (After Fonseca.) 



15. Isospora hominis. (After Dobell.) 



16. Eimeria clupearum (=£. wenyoni). (After Wenyon.) 



17. Eimeria sardime (= E. oxyspora). (After Dobell.) 



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