Longtime use of certain medications can cause drug-induced lupus, but scientists do not know what causes other types of lupus. Many suspect that genes and hormones predispose an individual to the disease, which may be triggered by certain environmental factors. Factors that influence a person's chances of developing lupus include:

• Sex and age. Ninety percent of lupus patients are women, usually in their childbearing years of 15 to 45, supporting the theory that female hormones play a role in role in the disease. However, early results from the landmark Carolina Lupus Study have unexpectedly not linked greater exposure to estrogen to increased risk of systemic lupus erythematosus (SLE), the most common and serious form of lupus.
  
  
• Race. Lupus is two to three times more common in people of color, according to the Lupus Foundation of America. The disease is typically more severe in blacks and Asians, according to the American College of Rheumatology (ACR).
  
  
• Genetics. Scientists have identified several genes as risk factors for lupus. Genes are believed to account for at least part of the racial disparity in prevalence of SLE. Researchers have identified a cluster of 14 genes, known as the interferon expression signature, in some SLE patients who develop severe complications such as kidney failure.
  
  
• Family history. About 5 percent of children develop lupus if a parent had it, and 20 percent of patients have a parent or sibling who has or may develop lupus, according to the Lupus Foundation of America.

There is much speculation about the possible environmental triggers of lupus. Workers exposed to crystalline silica (quartz) dust, a mineral found in rocks and soil, may have 2 to 4 times the risk of developing the disease, the National Institutes of Health reported in 2007, based on early results from the Carolina Lupus Study. Viruses, sunlight and stress have been implicated but not proven. According to the U.S. Food and Drug Administration (FDA), triggers in susceptible people may include:

• Infections such as the Epstein-Barr virus
• Ultraviolet light
• Medications such as sulfa antibiotics
• The herbal supplement echinacea (purple coneflower), which many people take in an effort to prevent or relieve colds