At present no one knows for sure. Scientists are presently engaged in research to see if they can isolate the gene or genes that make someone susceptible to Lupus. Lupus is NOT catching. It is a disorder of the immune system.
Presently, scientists believe that it is a combination of genes and environmental factors. Meaning that you are predisposed to Lupus and something in your environment triggers it...maybe stress, food allergies, etc.