Your lungs are covered with 2 thin layers of tissue called the pleura. One layer of tissue is on the lung and the other lines the chest cavity. In between these layers is a small space that is most often filled with a small amount of fluid. The fluid allows the 2 layers to glide over one another when you breathe in and out. Pleurisy happens when the layers of tissue or pleura become irritated as they rub against one another each time you breathe. This causes sharp pain when you breathe. Pleurisy can happen after a lung injury or infection. It may become a serious condition if not treated.