Human Immunodeficiency Virus (HIV) attacks the immune system. It stays in the body for life, but treatment can keep it under control and keep the immune system healthy. Without medication, people with HIV can develop AIDS (Acquired Immune Deficiency Syndrome). AIDS is the most advanced stage of an HIV infection, when the immune system can no longer fight infections. Someone with AIDS has both HIV and at least one of a specific list of “AIDS-defining” diseases, which includes tuberculosis, pneumonia and some types of cancer.