The human immunodeficiency virus (HIV) is a retrovirus that infects
cells of the human immune system, destroying or impairing their
function. In the early stages of infection, the person has no symptoms.
However, as the infection progresses, the immune system becomes weaker,
and the person becomes more susceptible to so-called opportunistic
infections.
The most advanced stage of HIV infection is
acquired immunodeficiency syndrome (AIDS). It can take 10-15 years for
an HIV-infected person to develop AIDS; antiretroviral drugs can slow
down the process even further.
HIV is transmitted through unprotected sexual intercourse
(anal or vaginal), transfusion of contaminated blood, sharing of
contaminated needles, and between a mother and her infant during
pregnancy, childbirth and breastfeeding.
More about HIV/AIDS
http://www.who.int/topics/hiv_aids/en/