The term AIDS refers to the most advanced stages of an infection caused by the human immunodeficiency virus (HIV). The current definition of AIDS includes:

• All HIV-infected people who have fewer than 200 CD4+ T cells (also called T4 cells or just T cells). These cells are key infection fighters in the body's immune system. Healthy adults usually have CD4+ T cell counts of around 600 or more.
• HIV-infected people who have been diagnosed with one or more health conditions that affect people with advanced HIV disease. These conditions include certain cancers. They also include certain infections that occur because of HIV's effect on the immune system (called opportunistic infections).

According to the CDC, about 1.5 million adults and teens are living with HIV infection in the U.S. Many more people are living with HIV infection outside the U.S., particularly in sub-Saharan Africa. The HIV epidemic is still not well-controlled in some parts of the world. But this is improving steadily as effective medicines have become more widely available.

AIDS is caused by the human immunodeficiency virus (HIV). HIV destroys or hurts immune system cells. It makes the body's ability to fight infections and certain cancers weaker. HIV is most often spread by having sex with an infected partner whose HIV is not diagnosed or is not under control. Another way to spread HIV is by having contact with infected body fluids, or with blood from contaminated needles, syringes, or other drug equipment.

HIV can't be cured. But it can be controlled. And much of the damage from the infection can be reversed or prevented. But if HIV is left untreated, serious infections and cancers occur because of the weakened immune system. And the virus can be passed on to others.

The risk of becoming infected with HIV depends on the type of activity. HIV is spread through blood, pre-seminal fluid and semen, fluids from the vagina and rectum, and breastmilk. So these types of behaviors put people at risk of getting or becoming infected with HIV:

• Having vaginal or anal sex with a person infected with HIV whose virus is not under control. Sexual contact is the most common way the virus is spread. The virus enters the body through the lining of the vagina, penis, rectum, or mouth during sexual activity. Unprotected anal sex has the highest risk of transmission.
• Having multiple sex partners. This includes any partners whose HIV status is not known. It also includes a partner who has HIV, but whose virus is not under control or is uncertain of the control of their HIV.
• Having high-risk sexual partners. This includes a partner who has a sexually transmitted infection (STI), is a sex worker, or has sex with many other people. Someone who injects or sniffs drugs is also a high-risk partner.
• Sharing needles, syringes, or other drug equipment with someone with HIV whose virus is not under control or someone with uncertain HIV status.

Other factors include:

• Using alcohol and other drugs. These can harm someone's judgment. They make it more likely that people will do risky things, such as having unprotected sex.
• Having a sexually transmitted infection. (STI) STIs can cause changes in the tissue of the vagina or penis. They can make it easier for HIV to pass while you're having sex. And a person with one STI is more likely to pick up another, including HIV.
• Having an accidental stick from a needle or medical device contaminated with HIV. But it is rare for a person with HIV to spread the virus to a healthcare worker this way. Or for a healthcare worker to spread the virus to a patient.
• Having a blood transfusion. HIV may also be spread through contact with infected blood. But the risk of getting HIV from blood transfusions is very low. This is because blood is screened for signs of HIV infection in the U.S.
• Having sex without knowing your partner's HIV status.

Risks to an unborn child. A mother infected with HIV can give her baby the virus before or during birth. This is especially true if her HIV is not well-controlled. She can also pass the virus by breastfeeding. Pregnant women should always be tested and treated for HIV.

Being exposed to these things does not put you at risk for getting HIV/AIDS:

• Saliva
• Sweat
• Tears
• Casual contact, such as sharing food utensils, towels, and bedding
• Swimming pools
• Telephones
• Toilet seats
• Biting insects (such as mosquitoes)

Many people develop a flu-like illness within 2 to 6 weeks after exposure to the HIV virus. But about 5 out of 10 people don't have any symptoms at all when they first become infected. In addition, the symptoms that do appear often go away within a week to a month. And they are often mistaken for those of another viral infection. These symptoms may include:

• Fever
• Headache
• General feeling of discomfort (malaise)
• Enlarged lymph nodes
• Sore throat
• Diarrhea
• Rash

Constant or severe symptoms may not show up for 10 years or more after HIV first enters the body in adults. In children born with an HIV infection, it may take 2 years for symptoms to appear. This period of no symptoms can be different for each person. But during this time, HIV is actively infecting and killing immune system cells and other cells throughout the body. Its most clear effect is a decrease in the number of CD4+ T cells. These cells are key infection fighters in the immune system.

As the immune system gets weaker, complications or symptoms begin to appear. Symptoms of advanced HIV disease and AIDS may be different for each person. Symptoms may include:

• Lymph nodes that stay enlarged for more than 3 months
• Lack of energy
• Weight loss
• Frequent fevers and sweats
• Constant or frequent yeast infections (oral or vaginal)
• Constant skin rashes or flaky skin
• Diarrhea that keeps coming back
• Short-term memory loss
• One or more infections (opportunistic infections) linked to having an immune system that is getting weaker. These include tuberculosis and certain types of pneumonia.

Some people develop frequent and severe herpes infections. These cause mouth, genital, or anal sores, or a painful nerve disease known as shingles. Children may have delayed development or slowed growth (failure to thrive).

During the course of the HIV infection, most people have a slow decline in the number of CD4+ T cells. Some people may have sudden and severe drops in these cell counts.

The symptoms of HIV infection may look like other health conditions. Always talk with your doctor for a diagnosis. Quick diagnostic tests are available, and early diagnosis is important.

Several types of HIV tests are used to diagnose HIV infection. Early HIV infection often causes no symptoms. It must be found by testing a person's blood for proteins (antibodies) made against HIV. Or the virus itself can be found. Tests used to find antigen-antibodies are a preferred method of testing. In this case, antigens are a portion of the virus. They are a sign of the active virus and also cause a response from the body's immune system. Antibodies are made by the body in an attempt to fight the virus and its antigens. Testing for antibodies or the virus is generally positive within 10 to 28 days after infection. But it may sometimes take longer, especially if an older HIV test (not the antigen-antibody test) is used. People exposed to HIV should be tested for HIV infection as soon as possible. In some cases, a person may have been very recently exposed or possibly exposed to HIV and that very early testing is negative. This is much less likely now with newer antigen-antibody testing. You may need to do repeat testing in 1 to 4 weeks. If your doctor is concerned about exposure to HIV in the last 10 to 14 days, they may also order another type of test to find the virus directly, called an HIV RNA test.

As with many other conditions, finding HIV early offers more chances for successful treatment. Antiretroviral medicines are a type of antiviral medicine for HIV that can stop the virus from further harming the body. This allows some or all of the damage to be healed. Antiretroviral treatment that fully controls the virus can also work well to prevent spreading the virus to others. People can then often live a normal lifespan and have a normal sex life and family life.

There is currently no cure for HIV infection. But people who take the medicines and stay on them may be able to keep the virus completely under control. Almost everyone with HIV infection will benefit from and needs anti-HIV (antiretroviral) treatment. Everyone whose HIV infection has progressed to AIDS needs antiretroviral treatment. Untreated AIDS will always lead to death. There is no other effective treatment besides antiretrovirals. Talk with your doctor for more information about various medicines for treating HIV/AIDS.

A lot of research is being done to find a vaccine that might either prevent HIV infection or help the body to better control HIV infection. Currently, no vaccine has been shown to work well enough to be used.

• AIDS is caused by the human immunodeficiency virus (HIV). HIV destroys or hurts immune system cells. It makes the body's ability to fight infections and certain cancers weaker.
• HIV is most often spread by having sex with an infected partner. It can also spread by having contact with infected body fluids or from blood from contaminated needles, syringes, or other drug equipment.
• A mother infected with HIV can give her baby the virus before or during birth. She can also pass the virus by breastfeeding.
• Many people have a flu-like illness within 2 to 6 weeks after exposure to the HIV virus. But about 5 out of 10 people don't have any symptoms at all at first.
• HIV can't be cured. But it can be well-controlled with antiretroviral medicines.

Tips to help you get the most from a visit to your doctor:

• Know the reason for your visit and what you want to happen.
• Before your visit, write down questions you want answered.
• Bring someone with you to help you ask questions and remember what your doctor tells you.
• At the visit, write down the name of a new diagnosis and any new medicines, treatments, or tests. Also write down any new instructions your doctor gives you.
• Know why a new medicine or treatment is prescribed and how it will help you. Also know what the side effects are.
• Ask if your condition can be treated in other ways.
• Know why a test or procedure is recommended and what the results could mean.
• Know what to expect if you do not take the medicine or have the test or procedure.
• If you have a follow-up appointment, write down the date, time, and purpose for that visit.
• Know how you can contact your doctor if you have questions.