How do you get autoimmune disease?

My friend at school said she has an autoimmune disease. Are autoimmune diseases contagious? How do you get an autoimmune disease?

Normally, the body’s immune system keeps us healthy and defends the body against foreign invaders (such as bacteria or viruses) that can cause illness. With an autoimmune disease, the body mistakenly attacks its own tissues.

The cause of autoimmune disease is unknown. Anyone can get an autoimmune disease but they are more common in women in their childbearing years, people with a family history of an autoimmune disease, certain environmental exposures (such as sunlight, chemical solvents, viruses, and bacteria), and certain races or ethic backgrounds (for example, African-Americans and Hispanic people are more likely to develop lupus and Caucasians are more likely to have type 1 diabetes).