HIV is the etiologic agent of AIDS. AIDS results from the loss of cell
s that are central to immune responses, T lymphocytes that express the
CD4 protein on their surface. This paper relates HIV structure and re
plication to the clinical course of HIV infection. The virology of HIV
replication is discussed first at the cellular and molecular levels.
The course of HIV infection in vivo then is discussed and related to H
IV replication. Finally, models that have been proposed to explain the
mechanism whereby HIV causes immunodeficiency are considered. Althoug
h much is known about the growth of the virus both in vitro and in viv
o, many questions remain about how HIV can deplete CD4-positive T lymp
hocytes and cause AIDS.