Pneumonia is an infection of the lung that affects 1 out of 100 people annually. Viral pneumonia can be caused by one of several viruses, including influenza, parainfluenza, adenovirus, rhinovirus, herpes simplex virus, respiratory syncytial virus, hantavirus, and cytomegalovirus.

Most cases of viral pneumonia are mild and get better without treatment, but some cases are more serious and require hospitalization. People at risk for more serious viral pneumonia typically have impaired immune systems and include HIV patients, transplant patients, young children (especially those with heart defects), the elderly, and people taking medications such as steroids to suppress their immune systems to treat autoimmune disorders.