This small volume is an excellent presentation of the current knowledge and approaches to the problem of parasite survival in the host which in many cases may also be immune to super-infection. There are thirteen major sections of the symposium. The first by way of introduction deals with the general immune response to parasites. This is followed by sections on the genetics of antigenic variation in protozoa and how antigenic variation relates to immunity to plasmodia, to African trypanosomes, and to its possible role in infection with the nematode Nippostrongylus. A general discussion of the variety of immunologic responses to infection and how these may be reflected in the clinical course of infection is followed by sections on suppression of immune responses in malaria infections and trypanosomiasis, and evasion of immune response in schistosomiasis by the acquisition of host antigens by the parasite. The possible role of soluble parasite antigens in blocking the immune response of the host is the subject of a subsequent section followed by a presentation which suggests that modulation of the immunopathologic aspects of schistosomiasis may be related to this phenomenon.