What Is It? The human immunodeficiency virus (HIV) weakens the
body's immune defenses by destroying CD4 (T-cell) lymphocytes,
which are white blood cells that normally help guard the body
against attacks by bacteria, viruses and other germs. When HIV
destroys CD4 cells, the body becomes vulnerable to many
different types of infections. These infections are called
"opportunistic" because usually they only have the opportunity
to invade the body when the immune defenses are weak. HIV
infection also increases the risk of certain cancers, illnesses
of the brain and nerves, body wasting, and death. The range of
symptoms and illnesses that can happen when HIV infection
severely weakens the body's immune defenses is called acquired
immunodeficiency syndrome or AIDS. Since 1981, when doctors
first recognized HIV/AIDS as a new illness, scientists have
learned much about how a person becomes infected with HIV. The
virus is spread through contact with an infected person's body
fluids, especially through blood, semen and