Around this time, healthcare providers in Los Angeles and New York were reporting rare cases of pneumonia, cancer, and other illnesses among male patients who had sex with other men. These conditions were not normally found in people with healthy immune systems. It was several years before scientists developed a test for the virus.

These are symptoms of HIV, but a blood test is the only way to know for sure if you have HIV infection. Symptoms are not always reliable. They can be mistaken for other illnesses. Severe symptoms like rapid weight loss or chronic infections usually don't appear for years. Even if you don’t have any symptoms, you can still infect other people. This is one important reason why early testing is so important.

An HIV-infected person has AIDS when they have fewer than 200 CD4 T-cells. These immune cells are an important part of the body’s ability to fight off infection When they fall too low, people with chronic HIV are at risk for over a dozen severe infections (opportunistic infections). Opportunistic infections don't occur in people with a normal immune system. A a person with HIV is also defined as having AIDS if they have an opportunistic infection. These include infections such as recurrent pneumonia and Kaposi sarcoma, a cancer-like disease affecting the skin. Other examples of opportunistic infections include toxo, crypt, and tuberculosis.

When you have HIV in your body, your immune system makes antibodies (a kind of protein) to fight the infection. These antibodies can be measured by a blood test. It usually takes 2 to 8 weeks after infection before HIV antibodies can be detected. For some people, it may take longer, but most people will develop antibodies within the first 3 months after infection. Once the antibodies are detected, a person is considered HIV-positive. A person can also be diagnosed with HIV infection when a blood test detects the actual virus particles.

HIV targets a kind of white blood cell called a CD4 positive T cell, or T4 cell. This type of cell is a key immune response cell and fighter of infections.

A normal count is 600 or more CD4 positive T cells per cubic milliliter of blood. Below 200 per cubic milliliter means a person infected with HIV has developed AIDS.

HIV is found mostly in blood, semen, or vaginal fluid. An HIV-positive person can pass the virus through unprotected sex (oral, vaginal, or anal). The HIV risk from these sexual activites varies widely. Receptive anal intercourse is the riskiest. The risk from oral sex is very low. They can also pass it on through sharing needles or syringes if their HIV is untreated or not fully controlled with medicines. Women whose HIV is not under control with medicines can pass the virus to their babies before birth, during birth, or through breastfeeding. The risk of getting HIV from a blood transfusion is extremely low. Since 1985, all donated blood in the U.S. is tested. HIV is not spread through casual contact, such as shaking hands or hugging, or from sharing food, glasses, utensils, towels, bedding, toilet seats, or pets.

The medicines are designed to attack the virus at different steps in its life cycle. When taken correctly, the cocktail blocks HIV from being active. It overwhelms any chance of developing resistance to the medicines.

Currently, no vaccine is available for HIV. Protect yourself by:

• Not having sex
• Staying in a sexual relationship with just one person (monogamous)
• Using a latex condom when you do have sex
• Taking PEP (post-exposure prophylaxis) or PrEP (pre-exposure prophylaxis) if you had unprotected sex with someone with HIV at high risk for HIV.
Taking PrEP (pre-exposure prophylaxis) every day if you are HIV negative and at risk for HIV. Risk factors include a look back into your sexual history over the last 6 months. If in the past 6 months, you have had unprotected vaginal or anal sex, if your partner is HIV-positive or may be HIV-positive, or if you yourself have had a sexually transmitted infection (STI), talk with your healthcare provider about PrEP.
• Getting checked for and treated for any other STIs you might have. STIs are also called sexually transmitted diseases (STDs).
• If you are having sex with someone who is living with HIV, making sure that they are taking their HIV medicines and their virus is "undetectable" for at least 6 months straight

Drug users must not share needles and syringes. They should not expose themselves to the blood of others. Remember, a person can be HIV-positive and not have symptoms. Have yourself tested to learn your status. Before you have sex with a new partner, ask the person about his or her HIV status. Ask when the person was last tested. Birth control pills and spermicides don’t protect you from getting HIV.