Below is an instruction from Human. Write a response.
    ### Instruction:
    What is (are) Bursitis ?
    ### Response:
    A bursa is a small, fluid-filled sac that acts as a cushion between a bone and other moving parts, such as muscles, tendons, or skin. Bursitis occurs when a bursa becomes inflamed. People get bursitis by overusing a joint. It can also be caused by an injury.
    