It is unknown. Some think there is a genetic gene that is found in peole w/ lupus.
Good question. Scientists are still working on that one. Some say genetics some say G-d is punishing them, others say it's a rare abnormal reaction to bacteria as a child. It's not really completely known. They have tracked some diseases and found a genetic component to them. But there could be more there on whether or not the person will ever become symptomatic. Some will test positive for the disease but it lays dormint their whole life. Awesome quesion though!