The Virus:

Influenza viruses are members of the Orthomyxoviridae family of viruses and are enveloped RNA viruses with eight segmented negative-strand RNA genomes. The genome is enclosed by the viral envelope which includes two major viral glycoproteins, the hemagglutin (HA) and the neuraminidase (NA), as well as the integral membrane viral protein (M2). The HA attaches the virus to host cell receptors and mediates fusion of viral and cellular membranes. The NA facilitates release of new viruses from the host cell while M2 proteins mediate viral uncoating. Each of the eight RNA genome segments is transcribed and replicated within the nucleus of the infected cells by a viral RNA-dependent RNA polymerase. The influenza viruses are characterized by mutability and high frequency of genetic reassortment. This leads to tremendous changes in the antigenic nature of the surface glycoproteins (NA and HA).

There are three types of influenza virus, A, B and C. Of these, type A virus has the highest antigenic variability resulting in its antigenic shift and antigenic drift, properties that pose significant problems for defense and development of treatments against this virus. Type C virus is antigenically most stable. Most cases of the flu, especially those that occur in epidemics or pandemics, are caused by the influenza A virus, which can affect a variety of animal species, but the B virus, which normally is only found in humans, is responsible for many localized outbreaks. The influenza C virus is least significant; it is generally non-symptomatic, and generally it is of little medical concern.