AIDS stands for Acquired Immune Deficiency Syndrome. It is the most advanced stage of infection caused by the Human Immunodeficiency Virus (HIV). AIDS occurs when the body's immune system is badly damaged by the virus, making the person vulnerable to other infections and diseases that the immune system would normally fight off effectively. A person with AIDS has a severely weakened immune system, which can lead to opportunistic infections and other serious illnesses. HIV is the virus that causes AIDS by attacking and destroying the immune cells that help the body fight infections. Most people with HIV do not develop AIDS if treated early with antiretroviral therapy (ART), which helps keep the immune system strong and controls the virus.