There are three distinct types of influenza virus, dubbed A, B, and C that have been identified. Together these viruses, which are genetically distinct from one another, comprise their own viral family, Orthomyxoviridae.
The structure of the influenza virus is somewhat variable, but the virus particles are usually spherical or ovoid in shape and 80 to 120 nanometres in diameter. The influenza virus is an enveloped virus that derives its lipid bilayer (a thin membrane made of two layers) from the plasma membrane of a host cell, a cell which harbous foreign molecules. Two different varieties of glycoprotein spike are embedded in the envelope. Approximately 80 percent of the spikes are hemagglutinin, a trimeric protein that functions in the attachment of the virus to a host cell. The remaining 20 percent or so of the glycoprotein spikes consist of neuraminidase, which is thought to be predominantly involved in facilitating the release of newly produced virus particles from the host cell. On the inner side of the envelope that surrounds an influenza virus is an antigenic matrix protein lining. Within the envelope is the influenza genome, which is organized into eight pieces of single-stranded RNA (A and B forms only; influenza C has 7 RNA segments). The RNA is packaged with nucleoprotein into a helical ribonucleoprotein form, with three polymerase peptides for each RNA segment.

Molecular Expressions 2005, cell biology and microscopy structure and functions of cells and viruses, Tampa, Florida, viewed 27 October 2009, <>

BY: Bishoy Aiad