My 13 yr old son is still undergoing testing for a diagnosis. The drs think he may possibly have a Rheumatological disease (Lupus, Stills, etc). He may be having a "flare up" presently. He came down with a virus and 2 days later his knees began to hurt. They have been hurting so severely now for a week he hasn't been able to walk on them. Can Lupus or any other Rheum. disease cause this?