Autoimmune diseases represent a family of at least 80 illnesses that share a common pathogenesis: an immune-mediated attack on the body's own organs. Collectively, autoimmune diseases affect some 20 million Americans, predominantly women. Many of these diseases are increasing in frequency in industrialized countries. Treatment of autoimmune diseases improved greatly during the second half of the 20th century but has been hampered because the diseases often progress before a clinical diagnosis is possible. Research in the 21st century is focusing on prospective prediction using the tools of systems biology.