One of the unique and most remarkable features of influenza virus is the frequency with which changes in antigenicity occur. Influenza virus infection is acquired by a mechanism involving transfer of virus containing respiratory secretions from an infected to a susceptible person. Viral replication occurs, followed by virus release and the host cell death. The occurrence of systemic illness and fever suggests dissemination of virus via the blood stream but infectious virus in the blood has only rarely been detected. The clinical findings of uncomplicated influenza infection characteristically begin with an abrupt onset after an incubation period of 1-2 days.
Many patients can pinpoint the hour of onset.
These symptoms usually persist for 3 days, the usual duration of fever. As the systemic signs and symptoms diminish, respiratory complaints and findings become more apparent. Cough is the most frequent and troublesome of these symptoms and may be accompanied by substernal discomfort or burning. The cough will persist for 2 weeks or so until the denuded respiratory epithelium is regenerated. Complications of influenza include primary influenza viral pneumonia.