A bursa is a closed, fluid-filled sac that functions as a gliding surface to reduce friction between tissues of the body. Bursae is plural for bursa. The major bursae are located adjacent to the tendons near the large joints, such as the shoulders, elbows, hips, and knees. When a bursa becomes inflamed, the condition is known as bursitis. Most commonly, bursitis is caused by local soft-tissue trauma or strain injury, and there is no infection (aseptic bursitis). On rare occasions, particularly when the immune system is suppressed, the bursa can become infected with bacteria. This condition is called septic bursitis.