128 PROTOZOAN PARASITISM 



animals but from each other. They are transmitted 

 from person to person more or less directly. Close 

 personal contact, faulty personal toilet hygiene, un- 

 sanitary disposal of human excrement, contamination 

 of water, milk, and food, exposure of excreta and food 

 to flies, all are factors in the spread of these parasites. 



Some of the species are very common, although 

 varying in incidence in different quarters. All are 

 of world-wide distribution. With the commonness 

 of Trichomonas, Chilomastix, and Giardia in the gen- 

 eral population of the world, it is readily appreciated 

 that a certain percentage of the sick who come under 

 the care of the medical profession will show one or 

 more of them. It may be conservatively estimated 

 that from twenty-five to fifty per cent of people in 

 any region will be found to harbor an intestinal fla- 

 gellate and that practically all people give tenantry 

 to one or more at some time. 



The fundamental fault, it seems, in most clinical 

 studies of "flagellosis" is the lack of control study. 

 It is recognized that proper scientific control of such 

 studies, which do not lend themselves to experimen- 

 tal confirmation, is most difficult. Having in mind 

 to compare a considerable group of flagellate hosts 

 to a similar group of non-parasitized, the writer 

 (1926) made an analysis of 1040 consecutive clinical 

 cases in whom gastro-intestinal studies had a prom- 

 inent place on account of the symptoms displayed, 

 the study being spread over a period of about four 

 years. 



