Aa Aa Aa Although most healthy adults who contract the flu experience relatively minor symptoms, influenza and other respiratory viruses are a serious health threat to the U.S. population at large.

Influenza is a common respiratory virus that is categorized into four types: A, B, C and D. Type C only causes mild infections and type D is not believed to infect humans. However, influenza A and ...

Box 1. Current nomenclature for influenza viruses (WHO 1980). The formal nomenclature for the designation of influenza viruses was revised and published by the WHO [43]. The year of isolation ...