Viruses may cause immunosuppression by a variety of mechanisms. This review delineates four categories. First, immunosuppression can result from the direct effects of viral replication on lymphocyte functions. Either all classes of lymphocytes can be affected, as occurs in measles, or the effect can be restricted to a cell subtype, as is the case with human T cell-lymphotropic virus type III. Second, the activity of soluble factors of viral or host origin released from infected cells can affect immunosuppression. A third mechanism results from viral infection of macrophages and affects the function of these cells in natural and acquired immunity. Finally, immunosuppression may result from viral triggering of an imbalance in immune regulation, which culminates in the overactivity of suppressor cells. A detailed knowledge of the mechanisms by which viruses are involved in immunosuppression may help in the design of strategies to reverse the effect.