The influenza viruses are the most common viral cause of pneumonia. Primary influenza pneumonia manifests with persistent symptoms of cough, sore throat, headache, myalgia, and malaise for more than three to five days. The symptoms may worsen with time, and new respiratory signs and symptoms, such as dyspnea and cyanosis, appear.
The reported incidence of viral pneumonia (see the image below) has increased during the past decade. In part, this apparent increase simply reflects improved diagnostic techniques, but an actual increase has also occurred. Depending on the virulence of the organism, as well as the age and comorbidities of the patient, viral pneumonia can vary from a mild and self-limited illness to a life-threatening disease.