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.
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), 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.

How can I tell if I have HIV?

The first symptoms of HIV infection can resemble symptoms of common cold or flu viruses. The symptoms of early infection can also be similar to the symptoms of other sexually transmitted diseases and other infections such as "mono" or hepatitis, which are much more commonly and more easily transmitted. Stress and anxiety can also produce symptoms in some people, even though they do not have HIV.
Some people who contract HIV experience very strong symptoms, but others experience none at all. Those who do have symptoms generally experience fever, fatigue, and, often, rash. Other common symptoms can include headache, swollen lymph nodes, and sore throat. These symptoms can occur within days or weeks of the initial exposure to the virus during a period called primary or acute HIV infection.
Because of the nonspecific symptoms associated with primary or acute HIV infection, symptoms are not a reliable way to diagnose HIV infection. Testing for HIV antibodies is the only way to know whether you have been infected; however, the HIV antibody test only works after the infected person's immune system develops antibodies to HIV. During the "window period" between the initial infection and the period in which antibodies are detectable (which can be from 2 weeks to 6 months, but is usually 3 months), standard HIV testing is ineffective.
If you are concerned that you may have recently acquired HIV and have symptoms described above, see a doctor. A doctor or other health care professional can help determine whether you may be infected with HIV or another infection. If HIV infection is suspected, he or she may perform a Polymerase Chain Reaction (commonly called "PCR") test to determine whether HIV is present in the blood.
Once the primary or acute infection is over, most people do not experience any visible symptoms for another 8-10 years. Left untreated, the immune system becomes increasingly weaker and the disease progresses to AIDS. The next symptoms experienced by individuals infected with the virus are often associated with the "opportunistic infections" that target individuals with AIDS such as pneumonia, tuberculosis, and toxoplasmosis.

How can I tell if someone else has HIV?

There is no way to know for sure if someone else has HIV. Many people with HIV look perfectly healthy. Other people who are sick with HIV may have symptoms that are identical to other common illnesses. You cannot tell by looking whether someone is HIV positive. The only way to know for sure is if someone tells you. It is important to consider how well you know someone and how much you trust them when talking about sex and HIV.