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

  • Respiratory syncytial virus (RSV appears mostly in children)

Though corona and rhinoviruses are quite common causes for the 'common cold' there are some forms that can produce more significant illness. Our ability to diagnose additional agents, coupled with increased spread of these agents, has added more viruses to the potential list:

  • SARS-CoV: Severe Acute Respiratory Syndrome coronavirus

  • MERS-CoV: Middle East Respiratory Syndrome coronavirus

  • HCoV-NL63: Human coronavirus NL63

  • HCoV-HKU1

  • HBoV: Human bocavirus

  • EV-D68: Enterovirus D68

Both SARS and MERS can produce severe respiratory infection with significant (10%) fatality rate. HCoV-NL63 HCoV-HKU1, HBoV, and EV-D68 usually cause a milder, self-limiting respiratory illness.

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.