A few people with HIV are described as nonprogressors. These people have HIV that does not progress to more severe symptoms or disease. Most nonprogressors:

• Have lived with the infection for 10 to 15 years and remained healthy.
• Do not have declining CD4+ cell counts.
• Have a very low level of HIV in their blood.

A small number of people never become infected with HIV despite years of exposure to the virus; for example, they may have repeated, unprotected sex with an infected person. These people are said to be HIV-resistant.

Studies are under way to determine why some people either do not become infected with HIV or, if they do, why they do not develop symptoms or lose CD4+ cells. Research has shown that:

• Some people's CD4+ cells are relatively resistant to HIV. If HIV cannot attach itself to CD4+ cells, it cannot destroy them.
• Some people's immune systems may be better able to destroy the virus.
• Some strains of HIV may not be as harmful.