infectious arthritis Septic arthritis (or septic arthritis) is a bacterial infection within the joints. Joints are usually germ-free. When bacteria enter a joint, it causes inflammation (swelling, redness) and pain. Infections usually only affect one joint: the knee, hip, wrist, shoulder, elbow, or ankle. Anyone can have an infection. together But it's most common in children under 3 and adults over 80. Untreated infectious arthritis can damage the joints. and led to joint replacement surgery.