Lupus, technically known as systemic lupus erythematosus, is an autoimmune disease in which the body's immune system mistakenly attacks healthy tissue in many parts of the body. Symptoms vary among people and may be mild to severe. Common symptoms include painful and swollen joints, fever, chest pain, hair loss, mouth ulcers, swollen lymph nodes, feeling tired, and a red rash which is most commonly on the face. Often there are periods of illness, called flares, and periods of remission during which there are few symptoms. The cause of SLE is not clear. It is thought to involve genetics together with environmental factors. Among identical twins, if one is affected there is a 24% chance the other one will be as well. Female sex hormones, sunlight, smoking, vitamin D deficiency, and certain infections are also believed to increase the risk. The mechanism involves an immune response by autoantibodies against a person's own tissues.