AIDS (acquired immune deficiency syndrome) isn't a disease in itself; rather, AIDS is a condition that develops when a person's body has been weakened by HIV (the human immunodeficiency virus). HIV is found in blood and sexual fluids and spreads mainly through unprotected sexual contact and the sharing of hypodermic needles and equipment.
When a person becomes infected with HIV, it damages his or her immune system, leading to immunodeficiency; the immune system can no longer fight off common germs and pathogens, so a person infected with HIV becomes ill from diseases that don't usually affect someone without HIV.
The most common way people are infected with HIV is by having sex with an infected person. You can't tell by looking at a person whether they have HIV, so you have to protect yourself -- and your sex partner.
and HIV Prevention
Don't have unprotected sex outside marriage or a committed relationship. If you or your partner has ever had unprotected sex -- or if either of you uses injected drugs -- the only way to be sure you don't have HIV is to get tested. Have two...
It can take HIV many years to damage the immune system enough to make the person vulnerable to these diseases, called opportunistic infections. These infections, including Kaposi's sarcoma, a form of skin cancer, take the opportunity to invade because they don't encounter resistance. When doctors see someone with one of these diseases, they know that HIV is probably responsible, and the person may be diagnosed with AIDS.
As HIV slowly invades a specific immune cell -- the CD4 T-cell -- HIV uses the immune cell's genetic material to reproduce itself and then kills the CD4 T-cell.
An HIV-infected person may not have any symptoms of disease during this time -- called the asymptomatic period. This can last 10 years or more for some people. During this time, the person's CD4 T-cell count is watched closely to guide treatment. The goal is to keep an infected person from advancing to AIDS. Once the CD4 T-cell count goes below 200, a person is diagnosed with AIDS.
The Worldwide HIV/AIDS Pandemic
The first AIDS case was documented in 1981, and HIV has since spread worldwide. In 2009, almost 2 million people died worldwide, and the epidemic continues to spread. Sub-Saharan Africa has the greatest number of people who are infected. The World Health Organization (WHO) and the United Nations' UNAIDS office estimate that over 33% of adults are infected with HIV in some areas of Africa. Millions of children have been orphaned. The epidemic is also growing rapidly in Eastern Europe and Asia. More than 34 million people worldwide are now living with HIV.