A number of viral infections induce profound alterations in immune function; these changes may or may not be related to direct infection of lymphoid cells. The immunologic abnormalities that occur during acute systemic viral infections such as measles or infectious mononucleosis include suppression of responses to tuberculin skin tests in vivo and suppression of mitogen responses in vitro, evidence of activation of the immune system with up-regulation of activation-associated cell-surface markers and soluble cellular products, and evidence of altered immune regulation with autoimmune disease and elevated IgE. These abnormalities are likely to be interrelated and can become chronic when the viral infection is not cleared and persists.