1.Spleen indices taken in southern Illinois indicate a substantial incidence of malaria in this section; the incidence of enlarged spleens is given.
2.The distribution of the disease in the southern part of the state is shown to be not uniform, but to follow certain characteristics of the physiography of the section. These characteristics are described, and on the basis of this finding the area of heaviest malaria incidence is delineated.
3.It is shown that within the area of heaviest infection the distribution of incidence is, generally speaking, not even and regular, but tends to be highly focal and localized usually in comparatively small areas. A large number of these foci have been localized on spot maps and the detailed data regarding them made available for future malaria control work.
4.In the hill sections of lower malaria incidence, there is shown an apparent relationship between the occurrence of malaria and proximity to a larger stream bed valley.
5.Certain observations in regard to the interpretation of spleen data generally are presented, and an illustration of one method of estimating the magnitude of a malaria problem in a limited area is presented.