Infection with the human immunodeficiency virus (HIV) weakens the body's immune system and increases the body's vulnerability to many different infections, as well as the development of certain cancers.

HIV is a viral infection that is primarily transmitted by sexual contact or sharing needles, or from an infected pregnant woman to her newborn. Negative antibody tests do not rule out recent infection. Most (95%) people who are infected will have a positive HIV antibody test within 12 weeks of exposure.

Although there are no specific symptoms or signs that confirm HIV infection, many people will develop a nonspecific illness 2 to 4 weeks after they have been infected. This initial illness may be characterized by fever, vomiting, diarrhea, muscle and joint pains, headache, sore throat, and/or painful lymph nodes. On average, people are ill for up to 2 weeks with the initial illness. In rare cases, the initial illness has occurred up to 10 months after infection. It is also possible to become infected with the HIV virus without having recognized the initial illness.

The average time from infection to the development of symptoms related to immunosuppression (decreased functioning of the immune system) is 10 years. Serious complications include unusual infections or cancers, weight loss, intellectual deterioration (dementia), and death. When the symptoms of HIV are severe, the disease is referred to as the acquired immunodeficiency syndrome (AIDS). Numerous treatment options now available for HIV-infected individuals allow many patients to control their infection and delay the progression of their disease to AIDS.