Autoimmune Disease. Autoimmune disease affects up to 50 million Americans, according to the American Autoimmune Related Diseases Association (AARDA) . An autoimmune disease develops when your immune system, which defends your body against disease, decides your healthy cells are foreign.As a result, your immune system attacks healthy cells.utoimmune Disease. Autoimmune disease affects up to 50 million Americans, according to the American Autoimmune Related Diseases Association (AARDA) . An autoimmune disease develops when your immune system, which defends your body against disease, decides your healthy cells are foreign.