HIV (human immunodeficiency virus) is a virus that attacks the immune system, the body's natural defense system. Without a strong immune system, the body has trouble fighting off disease. Both the virus and the infection it causes are called HIV.

White blood cells are an important part of the immune system. HIV infects and destroys certain white blood cells called CD4+ cells. If too many CD4+ cells are destroyed, the body can no longer defend itself against infection.

The last stage of HIV infection is AIDS (acquired immunodeficiency syndrome). People with AIDS have a low number of CD4+ cells. They get infections or cancers that rarely occur in healthy people. These can be deadly.

There are two types of HIV:

• HIV-1, which causes almost all the cases of AIDS worldwide.
• HIV-2, which causes an AIDS-like illness. HIV-2 infection is uncommon in North America.

But having HIV doesn't mean you have AIDS. Even without treatment, it takes time for HIV to progress to AIDS—usually 10 to 12 years. With treatment, many people with HIV are able to live long and active lives.