The interaction between parasites and their animal or human hosts is one of the most interesting and important biologic phenomena. Understanding the host-parasite interplay is essential to understanding the pathogenesis of parasitic diseases and for the development of new vaccines and chemotherapy. One approach to identifying key components of the host immune response to parasites is using immunodeficient host models. Scid mice are one of the most useful models of an immunodeficient host. Herein, we describe the use of scid mice to identify the steps in granuloma formation to parasite eggs, devise a screening model for vaccine development, and provide clues to the pathogenesis of opportunistic infections associated with AIDS.