An autoimmune disease is a disorder that arises from an unusual immune response of the body against healthy body tissue. 

Essentially, it is the body mistakenly attacking and destroying itself.  This may be restricted to certain organs or involve a particular tissue.  Immunosuppressants, medications that decreases the immune response, are typically the treatment for the more than 80 different types of autoimmune diseases.  Some examples of autoimmune diseases that may sound familiar are Celiac disease, thyroiditis, fibromyalgia, Lupus and rheumatoid arthritis.