A disease in which the immune system, naturally designed to protect the body from foreign invaders such as viruses and bacteria, instead turns against and causes damage to the body’s healthy tissues. The immune system is a complex organization within the body that is designed normally to "seek and destroy" invaders of the body, including infectious agents. Patients with autoimmune diseases frequently have unusual antibodies circulating in their blood that target their own body tissues.

Examples of autoimmune diseases include systemic lupus erythematosus, Sjogren syndrome, Hashimoto thyroiditis, rheumatoid arthritis, juvenile (type 1) diabetes, polymyositis, scleroderma, Addison disease, vitiligo, pernicious anemia, glomerulonephritis, and pulmonary fibrosis.

Autoimmune diseases are more frequent in women than in men. It is felt that the estrogen of females may influence the immune system to predispose some women to autoimmune diseases. Furthermore, the presence of one autoimmune disease increases the chance for developing another simultaneous autoimmune disease.