After contamination, most respiratory viruses tend to multiply in the epithelium of the upper airway and secondarily infect the lung by means of airway secretions or hematogenous spread. Severe pneumonias may result in extensive consolidation of the lungs with varying degrees of hemorrhage. Some patients developed bloody pleural effusions and diffuse alveolar damage.[6]
The mechanism of damage to tissues depends on the virus involved. Some viruses are mainly cytopathic, directly affecting the pneumocytes or the bronchial cells. With others, overexuberant inflammation from the immune response is the mainstay of the pathogenic process.