HIV is a retrovirus that can lead to acquired immunodeficiency syndrome (AIDS), a condition in humans in which the immune system begins to fail, leading to life-threatening opportunistic infections.  Previous names for the virus include human T-lymphotropic virus-III (HTLV-III), lymphadenopathy-associated virus (LAV), or AIDS-associated retrovirus (ARV).

The Major Routes of Transmission:

1. Unprotected sexual intercourse
2. Contaminated needles
3. Transmission from an infected mother to her baby at birth
4. Transmission from an infected mother to her baby through breast milk

Screening of blood products for HIV in the developed world has largely eliminated transmission through blood transfusions or infected blood products.

HIV infection in humans is now pandemic.  As of January 2006, the Joint United Nation Program on HIV / AIDS (UNAIDS) and the World Health Organization (WHO) estimate that AIDS has killed more than 25 million people since it was first recognized on December 1, 1981, making it one of the most destructive pandemics in recorded history.  In 2005 alone, AIDS claimed an estimated 2.4–3.3 million lives, of which more than 570,000 were children.  It is estimated that about 0.6% of the world's living population is infected with HIV. HIV primarily infects vital cells in the human immune system such as helper T cells (specifically CD4+ T cells), macrophages and dendritic cells.

HIV infection leads to low levels of CD4+ T cells through three main mechanisms:

1. Direct viral killing of infected cells
2. Increased rates of apoptosis in infected cells
3. Killing of infected CD4+ T cells by CD8 cytotoxic lymphocytes that recognize infected cells