We are concerned in this and the following two chapters with the formulation and study of transmission models for malaria, schistosomiasis, and hermaphroditic helminthiasis. Any transmission model can be viewed as a model for the population of parasites in an ecological community. The size of the parasite population is an indication of the severity of infection in the community. A basic purpose of a transmission model is to find how the size of the parasite population is determined by biological and environmental influences. Once this purpose has been achieved, the model can also be used to evaluate the efforts required for eradication and the efficiencies of various modes of control.