Pleurisy is a condition of the pleura involving swelling. The pleura is a large, thin sheet of tissue that wraps around the outside of your lungs and lines the inside of your chest cavity. Between the layers of the pleura is a very thin space that is normally filled with a small amount of fluid. The fluid helps the two layers of the pleura glide smoothly past each other as your lungs breathe air in and out. Pleurisy is an inflammation of the pleura that causes sharp pain with breathing. The most common cause of pleurisy is viral infection. Treatment involves removing the fluid, air or blood from the pleural space to provide comfort and treating the underlying condition.