AIDS is a chronic, life-threatening condition caused by the human immunodeficiency virus (HIV). By damaging or destroying the cells of your immune system, HIV interferes with your body's ability to effectively fight off viruses, bacteria and fungi that cause disease. This makes you more susceptible to opportunistic infections your body would normally resist, such as pneumonia and meningitis, and to certain types of cancers.

The virus and the infection itself are known as HIV. The term AIDS (acquired immunodeficiency syndrome) is used to mean the later stages of HIV infection. But both the terms HIV and AIDS refer to the same disease.

HIV is most commonly spread by sexual contact with an infected partner. It can also spread through infected blood and shared needles or syringes contaminated with the virus. Untreated women with HIV also can pass the infection to their babies during pregnancy, delivery or through their breast milk.

In the two decades since the first reports of the disease, AIDS has become a global epidemic. Worldwide, an estimated 40 million people are living with HIV, including an estimated 2.5 million children younger than 15. According to the United Nations Program on HIV/AIDS (UNAIDS) and the World Health Organization (WHO) , 5 million people were newly infected with HIV in 2003 and 3 million people died from AIDS.

In sub-Saharan Africa, the area of the world most severely impacted by AIDS, 3 million new infections occurred in 2003, and there were 2.3 million AIDS deaths. The AIDS epidemic is also growing fast in China, India, Indonesia, Russia, Eastern Europe and Central Asia.

Currently, an estimated 850,000 to 950,000 Americans are living with HIV/AIDS. This is partly the result of improved treatments. Since 1995 the number of medications available to treat AIDS has greatly increased, and powerful combinations of newer antiretroviral drugs have helped reduce serious complications of the disease and prolong life. But the emergence of drug-resistant forms of HIV threatens the positive news about treatment.

Of equal concern is a growing public complacency about AIDS. Nearly a third of the people living with HIV don't know they're infected and so are more likely to spread the disease. And reports from several cities in both the United States and Europe show increased high-risk behavior among young gay men. Drug use is also fueling the spread of HIV here and abroad. These facts have led experts to warn that the 20-year-old epidemic is still in its early stages.