Influenza infections have caused significant morbidity and mortality throughout recorded human history. Each year, seasonal influenza is estimated to cause around 500 000 excess deaths worldwide. At least 32 pandemics have occurred since 1580 in addition to the seasonal waves of influenza infection. In 1918, a particularly severe pandemic, dubbed the “Spanish flu,” led to the rapid spread of influenza and resulted in at least 20 million deaths worldwide. Recently, the possibility of a worldwide pandemic due to pathogenic avian influenza viruses has become of great concern. Significant resources worldwide have been dedicated to the detection and containment of influenza outbreaks and the development of response plans to influenza epidemics at international, national, and local levels.

Influenza viral structure

Influenza viruses are enveloped, single-stranded, negative-sense RNA viruses in the family Orthomyxoviridae, which include the genera Influenzavirus types A, B, and C. Influenza tends to be spherical and 80 to 120 nm in diameter. Influenza A and B viruses cause the majority of human infections with influenza, whereas influenza C virus causes only sporadic upper respiratory infections. Influenza A viruses can cause infections in birds as well as humans, swine, and other mammals.

Influenza virus has an envelope composed of a lipid bilayer, with a layer of matrix protein on the inner surface and spike-like surface projections of glycoproteins on the outer surface. These glycoproteins include hemagglutinin and neuraminidase.