Autoimmune diseases are a collection of diseases that occur when the body’s own defenses — the immune system — attack healthy body tissues. Normally the immune system acts to defend the body from foreign invaders such as viruses and bacteria; in someone with an autoimmune disease, the immune system mistakenly attacks normal body proteins, a process called autoimmunity, which damages otherwise healthy cells and tissues.

There are at least 15 diseases known to be caused by autoimmunity and more than 65 others that are suspected, but not yet conclusively proven, to be of autoimmune origin. These diseases affect virtually every site in the body; the particular symptoms of each disease depend on which body tissues are attacked by the immune system (e.g. rheumatoid arthritis attacks the joints while multiple sclerosis attacks the central nervous system). However, several autoimmune diseases have overlapping symptoms, which can make them more difficult for a doctor to diagnose. Some of the most common autoimmune diseases include rheumatoid arthritis (RA), multiple sclerosis (MS), systemic lupus erythematosus (also known as lupus or SLE), type 1 diabetes mellitus, Graves’ disease, Hashimoto’s thyroiditis, and ulcerative colitis.

Many autoimmune diseases are characterized by tendency of the symptoms to wax and wane over time. While the diseases do not usually go away entirely, and there are no cures for many of them, in most cases the symptoms can be successfully treated.

Most autoimmune diseases are relatively rare, but collectively they affect an estimated 5 to 8 percent of the U.S. population (between 15 and 24 million Americans). Autoimmune diseases tend to run in families and women are nearly three times as likely as men to develop an autoimmune disease; an estimated 78% of all autoimmune disease sufferers are women.