Autoimmune Disease

One job of the immune system is to protect the body from viruses, bacteria, and other living organisms. Usually, the immune system does not react against the body's own cells. However, sometimes it mistakenly attacks the cells it is supposed to protect. This response is called autoimmunity. Researchers speculate that certain bacteria, viruses, toxins, and drugs trigger an autoimmune response in people who are genetically susceptible to developing an autoimmune disorder.

source: http://digestive.niddk.nih.gov/ddiseases/pubs/autoimmunehep/index.htm