Septic arthritis is a bacterial infection in a joint. Common joints include the knee, elbow and ankle. It may be caused by bacteria entering the joint either through the bloodstream, after a penetrating injury or following an infection of a surrounding bone. Often the cause is unknown. The infection causes the build up of fluid or pus within the joint (effusion). This fact sheet describes the signs and symptoms of this condition, diagnosis and treatment, in addition to providing valuable additional resources.