In the event that the virus infects the lungs, loss of alveolar epithelium allows plasma to leak from capillaries into the alveolar space. This is especially problematic in patients predisposed to pulmonary edema. Pulmonary edema can be caused by any of the following underlying conditions: (1) increased capillary hydrostatic pressure, (2) increased capillary permeability, (3) decreased plasma oncotic pressure, and (4) lymphatic obstruction.
Loss of alveolar epithelium and the resulting pulmonary edema predispose the underlying tissue to colonization by bacteria. Secondary bacterial infection can lead to pneumonia, which accounts for most influenza related deaths.