Bursitis is a condition that causes inflammation of a bursa, which is a closed, fluid-filled sac that works as a cushion and gliding surface to reduce friction between tissues of the body. The most common causes of bursitis are repetitive motions or positions that put pressure on the bursae around a joint, such as throwing a baseball or lifting something over your head repeatedly, leaning on your elbows for long periods, or extensive kneeling for tasks such as laying carpet or scrubbing floors. Other causes include injury or trauma to the affected area, inflammatory arthritis such as rheumatoid arthritis, gout, tendonitis, diabetes, and thyroid disease, and infection. Risk factors for bursitis include aging, certain occupations or hobbies that require repetitive motion or pressure on particular bursae, and certain health conditions.