The human immunodeficiency virus (HIV) kills or impairs cells of the immune system and progressively destroys the body's ability to fight infections and certain cancers. HIV is most commonly spread by sexual contact with an infected partner. The term AIDS (acquired immune deficiency syndrome) applies to the most advanced stages of an HIV infection. Today, there are medical treatments that can slow down the rate at which HIV weakens the immune system, but currently there is no cure for the disease. However, there are other treatments that can prevent or cure the conditions associated with AIDS. Clinical trials expand knowledge and treatment options.