Here is the microscopic appearance of a viral pneumonia with interstitial lymphocytic infiltrates. Note that there is no alveolar exudate. Thus, the patient with this type of pneumonia will probably not have a productive cough. The most common causes for viral pneumonia are influenza A and B, parainfluenza, adenovirus, human metapneumovirus, and respiratory syncytial virus (RSV appears mostly in children). Cytomegalovirus can appear in immunocompromised hosts.
Influenza A viral virulence varies considerably, as the virus undergoes genetic drift and shift with recombinations of its segmented RNA genome, particulary with shift when there are cross species shift, most often involving birds and swine. Examples include: H1N1, H1N5, H7N9.