HIV stands for Human Immunodeficiency Virus. HIV is a virus. Some viruses, such as the ones that cause the common cold or the flu, stay in the body only for a few days. Some viruses, such as HIV, never go away. When a person becomes infected with HIV, that person becomes "HIV positive" and will always be HIV positive. Over time, HIV disease infects and kills white blood cells called CD4 lymphocytes (or "T cells") and can leave the body unable to fight off certain kinds of infections and cancers. Infections that occur due to HIV are called opportunistic infections because they take advantage of the impaired immune system.
With successful antiretroviral therapy (ART), the body can remain healthy and fight off most viruses and bacteria. A healthy person usually has a CD4 count of between 600 and 1,200. When the CD4 count drops below 200, a person's immune system is severely weakened, and that person is then diagnosed with AIDS, even if he or she has not become sick from other infections.
AIDS stands for Acquired Immune Deficiency Syndrome and is caused by HIV. The names HIV and AIDS can be confusing because both terms describe the same disease. Think of AIDS as advanced HIV disease. A person with AIDS has an immune system so weakened by HIV that the person usually becomes sick from one of several opportunistic infections or cancers such as PCP (a type of pneumonia) or KS (Kaposi sarcoma, a type of cancer that affects the skin and internal organs in HIV), wasting syndrome (involuntary weight loss), memory impairment, or tuberculosis. If someone with HIV is diagnosed with one of these opportunistic infections (even if the CD4 count is above 200), he or she is said to have AIDS. AIDS usually takes time to develop from the time a person acquires HIV--usually between 2 to 10 years or more.
Once a person has been diagnosed with AIDS, she or he is always considered to have AIDS, even if that person's CD4 count goes up again and/or they recover from the disease that defined their AIDS diagnosis.