Cell mediated and humoral immune protection against virus infections are discussed in relation to development and use of recombinant vaccines in the murine lymphocytic choriomeningitis virus infection. In the same model, immunopathological aspects of T cell immunity to viruses are examined in the murine lymphocytic choriomeningitis virus infection. This virus causes an acquired immunosuppression that is not caused by the virus itself but by cytotoxic antiviral T CD8+ effector T cells which destroy virus infected macrophages and antigen presenting cells. This paradoxical immunopathological mechanism may apply partially to HIV induced immunosuppression in humans.