AIDS (Acquired Immuno Deficiency Syndrome) is a viral disease of immune system. It is caused by virus Human Immunodeficiency Virus (HIV). In AIDS patients, the virus homes on CD4+ T-cells (helper T-lymphocytes), infecting and killing them until none are left. Without their crucial immune system cells, the body cannot mount a defence against invading bacteria or viruses. AIDS patient die off infections that a healthy person could fought off.