- Investigations on the effect of virus infections on the host’s immune system started almost twenty years ago when Peterson et al. demonstrated that Gross virus-infected mice are impaired in their ability to respond in antibody production to the T2 bacteriophage (1). Similar findings were reported in animals infected with other oncogenic viruses, such as Moloney, Friend or Rauscher leukemia viruses (2–4). Viruses which do not cause leukemia but infect lymphoid tissue were also found to modulate immune responses. Thus Aleutian mink disease virus (5), lymphocytic choriomeningitis virus (6) and M-P virus (7) were found to degress the host humoral responses to sheep erythrocytes (SRBC), while Venezuelan equine encephalitis virus (8) and lactic dehydrogenase virus (LDV; 9–14) were found to enhance antibody responses to human gamma globulin (HGG) or SRBC.