Pneumonia is an inflammatory illness of the lung. Frequently, it is escribed as lung parenchyma/alveolar inflammation and abnormal alveolar filling with fluid. (The alveoli are microscopic air-filled sacs in the lungs responsible for absorbing oxygen from the atmosphere.) Pneumonia can result from a variety of causes, including infection with bacteria, viruses, fungi, or parasites, and chemical or physical injury to the lungs. Its cause may also be officially described as idiopathic—that is, unknown—when infectious causes have been excluded.