Bursitis is the swelling and irritation of a bursa. A bursa is a fluid-filled sac that acts as a cushion between a muscles, tendons, and bones. Bursitis is often a result of overuse. It can be caused by a change in activity level, such as training for a marathon or by being overweight. Bursitis can also be caused by trauma, rheumatoid arthritis, gout, or infection. Sometimes the cause cannot be found. Bursitis commonly occurs in the shoulder, knee, elbow, and hip. Other areas that may be affected include the Achilles tendon and the foot.