Influenza virus

Influenza viruses are unique among the respiratory viruses with regard to their frequent antigenic changes, seasonality, and impact on the general population. They can cause explosive outbreaks of febrile respiratory illness across all age groups and often substantial mortality, particularly in aged and chronically ill persons. Epidemics resembling influenza have been recorded since antiquity. The plague of Athens in 430 to 427 BC, described by Thucydides, has been postulated to have been due to epidemic influenza complicated by toxigenic staphylococcal disease (1). The greatest effects of influenza are seen when novel strains, to which most persons are susceptible, cause worldwide outbreaks, or pandemics. The most profound of these in modern times was the 1918 pandemic that may have claimed as many as 100 million lives worldwide (2). Sequencing of RNA fragments from tissue samples taken from 1918 pandemic victims enabled reconstruction of the extinct 1918 virus and study of its virulence in animal models (3, 4).