Pneumonia is an acute infection of the lung parenchyma that commonly impairs gas exchange.

In bacterial pneumonia, an infection triggers alveolar ianflammation and edema. This produces an area of low ventilation with normal perfusion. Capillaries become engorged with blood, causing stasis. As the alveocapillary membrane breaks down, alveoli fill with blood and exudates, resulting in atelectasis. In viral pneumonia, the virus attacks bronchial epithelial cells, causing ianflammation and desquamation. The virus also invades mucous glands and globet cells, spreading to the alveoli, which fill with blood and fluid.