* immune system is the system

^* immune system is the system of the body compose d of specialized cells and the substances they produce that helps protect the body against disease-causing germs.

Infection with HIV takes about 10 years to develop into full-blown AIDS. During most of this period, people usually look and feel healthy

The HIV virus attacks T-helper cells, or CD4+ cells, by fitting itself into the cell like a key in a lock. Once it has invaded, it can use its own RNA as a template to make copies of itself, multiplying and traveling through the body. This process destroys the body's own T cells over time; as the T-cell count falls, the body's resistance to germs and disease declines.

Worldwide, about one-third of people with HIV/AIDS are between 15 and 24 years of age. Teens and young adults are at particular risk because they are more likely to have unprotected sex.

and are not aware that they are infected unless they are tested for the virus. Currently, experts think that, untreated, people with HIV infection eventually will develop AIDS unless they die first of other causes. Medications for HIV infection improved greatly in the 1990s, and the life expectancy of people receiving proper treatment has been extended dramatically. However, none of these medicines actually cure the infection.

AIDS was first described in the United States in 1981, after many gay men in San Francisco and New York City became ill from an unknown infectious cause. In 1985, it became clear that a retrovirus causes AIDS.