48 PROTOZOAN PARASITISM 



tion. In temperate or colder regions its incidence is 

 usually less, but it has been found wherever search has 

 been made for it. The reported incidence in different 

 surveys varies widely, from around 1% to 20% and 

 more, the variation in reliable studies apparently de- 

 pending largely on the nature of the examined people, 

 as to the sanitation of their environment and whether 

 they are well or subjects of intestinal disease. The 

 greater incidence is naturally where the protection of 

 man from the excreta of his kind is poorest^ this 

 usually being the case in tropical and subtropical 

 regions. In institutional life, with its close personal 

 contact, it is likely to be comparatively high. In 

 rural districts without modern facilities for sewage 

 disposal it is also apt to be high. In modern com- 

 munities with regulated sewage disposal, care of milk 

 and other food, and water, it is low. Through a 

 country like the United States it will probably aver- 

 age less than 10%. In the experience of the writer 

 the incidence in the population of the modern city 

 and town is nearer 3%. The food handler carrier 

 and close personal contact with infected persons are 

 undoubtedly the main means of spread of the parasite 

 among the inhabitants of districts of good sanitary 

 facilities. Prolific spread and the production of epi- 

 demics of disease by it are not at all likely, although 

 Craig (1026) shows that epidemics in special groups, 

 such as troops, exposed to massive infection under 

 certain conditions may occur. The concern of some 

 about the danger to the people of this country from 



