The exact cause of lupus isn't known. Experts believe that some people are born with certain genes that affect how the immune system works. These people are more likely to get lupus.

A number of other things can trigger lupus attacks. These include:

• Viral infections, including the virus that causes mononucleosis.
• Some medicines.
• Some chemicals.
• Tobacco smoke.
• Sunlight.

Although these things can trigger lupus, they may affect one person but not another person.