Separate worlds set to collide: smallpox, vaccinia virus vaccination, and human immunodeficiency virus and acquired immunodeficiency syndrome.

Concerns about the possible release of smallpox by bioterrorists has led to policies that recommend smallpox vaccination of some health care providers, and, in the near future, the vaccine may become available to the general population on a voluntary basis. Both smallpox virus (variola virus) and the smallpox vaccine (vaccinia virus) will have a significant impact on people infected with human immunodeficiency virus (HIV). Given that populations with acquired immunodeficiency syndrome and populations with immunosuppressed conditions due to solid organ and bone marrow transplantation were not present in the days when smallpox was prevalent, we will speculate on how smallpox might present in immunodeficient patients, and we will review the adverse events expected from the smallpox vaccine in hosts with HIV infection.