Septic arthritis is an infection in a joint. The infection can be caused by bacteria, viruses, or, less frequently, by fungi or parasites. Usually the infection involves a single large joint, such as the knee, but many joints may be involved. The initial infectious process can begin elsewhere in the body and travel through the bloodstream to the joint. Other sources include open wounds, surgery, and unsterile injections. Young children and elderly individuals are most likely to develop joint infections. The April 4, 2007, issue of JAMA includes an article that categorizes clinical findings that help identify patients who may have septic arthritis.