AIDS

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.