H.I.V. (human immunodeficiency virus) is a virus that kills the CD4 cells, or T-helper cells, which help the body fight infections. Once the body's immune system has been compromised, a person develops AIDS (acquired immunodeficiency syndrome).

H.I.V. is found in bodily fluids, and it can be transmitted when these come into contact with an uninfected person’s broken skin or mucous membranes. The most common means of transmission by far are:

• Sexual contact with an infected person
• Sharing needles or syringes (primarily for drug injection) with someone who is infected
• During pregnancy or birth to babies of HIV-infected mothers, or through breast-feeding after birth
Although additional transmission possibilities exist – such as accidental skin puncture by infected-needles in medical settings; introduction of infected blood into a caregiver's open cut (or eyes or nose); contact with infected blood by sharing razors or toothbrushes in a household setting – they are very rare. Insect bites and contact with H.I.V.-infected sweat have also been studied, but the risks have been found to be essentially non-existent. In fact, studies have shown that simply drying H.I.V.-infected fluids reduces viral presence by 90-99%.

Although improved treatments are helping some AIDS patients lead longer, healthier lives, there is no vaccine for H.I.V. and no cure for AIDS.