Lupus (systemic lupus erythematosus, or SLE) is an autoimmune disease, in which the immune system attacks the body's own tissues as though they were foreign substances. Lupus is not contagious.

No one knows exactly what causes the body to attack its own tissues. A person may be born with a certain genetic makeup that affects how the immune system functions or makes him or her at risk for lupus. A combination of factors can trigger the autoimmune process, some of which may affect one person but not another.

• Exposure to ultraviolet light, usually from sunlight, is known to trigger the disease process and symptom flares.
• Hormonal factors are linked to autoimmune disease, though the link is poorly understood. Some research shows an increased risk of lupus with higher levels of estrogen. But a separate study did not find evidence that the hormones estrogen or prolactin taken for hormone replacement therapy or birth control, or taken previously for fertility treatments, increased the risk of lupus. A more recent study showed that women with stable lupus had no increased risk of symptom flares from taking birth control pills.
• Smoking may increase the risk of getting lupus and may make the disease more severe.
• Some medications are suspected triggers of lupus and symptom flares.
• Some infections are suspected triggers. Some people who have cytomegalovirus (CMV), parvovirus (such as fifth disease), and hepatitis C infections eventually develop lupus. The Epstein-Barr virus has been linked to lupus in children.
• Chemical exposure has been known to trigger lupus. Suspected chemical toxins include trichloroethylene in well water and silica dust. Hair dyes and straighteners, linked to lupus in the past, are no longer considered to be lupus triggers.