AIDS (Acquired Immune Deficiency Syndrome) is a disease caused by HIV (the Human Immunodeficiency Virus). This is a particularly problematic disease for the immune system because the virus actually attacks immune system cells. In particular, it reproduces inside Helper T cells and kills them in the process. Without Helper T cells to orchestrate things, the immune system eventually collapses and the victim dies of some other infection that the immune system would normally be able to handle. See How AIDS Works as well as the links below for more information.