The life cycle of T. cruzi involves passage through a hematophagous insect host which transmits the infection to humans and other mammals through the process of feeding. Humans also acquire infection via blood transfusion or transplacentally. Trypanosoma cruzi is a flagellate protozoan of major medical importance. It causes Chagas’ disease, which is a major health problem in many Latin American countries. It is a leading cause of infectious heart disease. The factors determining whether chronic disease will develop, or whether the infection will persist in an indeterminate form, are not known. It is clear from animal models that both parasite and host differences may influence the outcome of infection. The ability of B cells or serum to passively confer resistance to acute disease appears to depend somewhat on how immune status was achieved in the donors. Once inside the host macrophage, the parasite transforms to the amastigote stage.