Human immunodeficiency virus (HIV) is a viral infection that impairs the body’s immune system. It causes AIDS, or acquired immune deficiency syndrome. AIDS means that the immune system has been seriously damaged. HIV can damage your immune system so you can't fight off other infections and diseases. Most people with HIV eventually develop AIDS, which is the most advanced stage of HIV infection. Despite the absence of a cure, patients with HIV infection without other significant medical conditions who are treated appropriately can expect to have a life expectancy, the same as the general population. AIDS is a syndrome, a group of symptoms that collectively indicate a disease. In the case of AIDS, symptoms can include the development of certain new infections and cancers. AIDS ultimately causes death. HIV infection can be transmitted through: Sexual contact. HIV is spread most commonly by sexual contact with an infected partner. The virus enters the body through the lining of the vagina, vulva, penis, rectum, or mouth during sexual activity. You can help protect yourself from infection by using a latex condom during sex and limiting sexual partners to those you know have no sexually transmitted diseases or HIV. A new drug strategy called PrEP, when combined with other safe-sex measures, can prevent HIV infection. Blood contamination. HIV may be spread through contact with infected blood. However, due to the screening of blood for evidence of HIV infection, the risk of acquiring HIV from blood transfusions is extremely low. Needles. HIV is frequently spread by sharing needles, syringes, or drug use equipment with someone who is infected with the virus. Transmission from patient to health care worker, or vice-versa through accidental sticks with contaminated needles or other medical instruments, is rare. Between mother and baby. HIV also can be spread to babies born to, or breast-fed by, mothers infected with the virus. HIV cannot be spread through saliva, sweat, tears, casual contact such as sharing food or utensils, swimming pools, telephones, toilet seats, and biting insects. Early HIV infection often causes no symptoms. Some people may develop a flulike illness within a month or two after exposure. But the symptoms usually disappear within a week to a month and are often mistaken for those of another viral infection. Persistent or severe symptoms may not surface for 10 years or more after HIV first enters the body in adults, or within two years in children born with an HIV infection. This asymptomatic period of the infection is highly variable from person to person. During this period, HIV is actively infecting and killing cells of the immune system. Its most obvious effect is a decline in the blood levels of CD4+ T cells (also called T4 cells)—the immune system's key infection fighters. The virus initially disables or destroys these cells without causing symptoms. As the immune system deteriorates, complications begin to surface, leading to AIDS. Diagnosing HIV is done by testing a person's blood for the presence of antibodies (disease-fighting proteins) to HIV. However, these HIV antibodies generally do not reach levels high enough to detect by standard blood tests until one to three months following infection, and it may take as long as six months. People exposed to HIV should be tested for HIV infection as soon as they are likely to develop antibodies to the virus. When a person is highly likely to be infected with HIV and yet antibody tests are negative, a test for the presence of HIV itself in the blood is used. Repeat antibody testing at a later date is also often recommended. Currently there is no cure for HIV. But early detection offers more treatment options that can slow down the rate at which HIV weakens the immune system. There are also treatments that can prevent or cure the conditions associated with AIDS.