My grandson is in the hospital now, he has had 2 knee surgerys to drain the infection out of his left knee,there is no known cause, no accident etc. he is 6 yrs. old, and the anti biotics has not helped, after many tests they have found a positive on the staph infection, The surgeon says we will probably never know the cause , they are switching anti biotics around., He will have to have a iv for 3 weeks after leaving the hospital. it is a mystery.