The virus is divided into three main types (A, B and C), which are distinguished by differences in two major internal proteins.


A schematic representation of an influenza A virus virion (the extracellular infective form of the virus). Each of the eight strands of RNA is labelled with the protein or group of proteins for which it is known to code, and some important protein functions are indicated. The genome of type B also comprises eight strands of RNA, whereas type C has seven strands [1,57]. Types A and B can be distinguished by differences in the internal proteins NP (nucleoprotein) and M1 (one of the matrix proteins denoted together by M on strand 7). In total, the eight segments of influenza A RNA encode 11 known gene products, including the recently discovered PB1-F2 protein [58]. The surface proteins HA (hemagglutinin) and NA(neuraminidase) are the principal targets of the humoral immune response (i.e. the response involving antibodies). Subtypes of influenza A are distinguished by differences in HA and NA. [Adapted from [59]. Copyright (2001) American Association for the Advancement of Science.]

Influenza virus type A is the most significant epidemiologically and the most interesting from an ecological and evolutionary standpoint, because it is found in a wide variety of bird and mammal species and can undergo major shifts in immunological properties. Type B is largely confined to humans and is an important cause of morbidity. Little is known about type C, which is not an important source of morbidity. Influenza A is further divided into subtypes based on differences in the membrane proteins hemagglutinin (HA) and neuraminidase (NA), which are the most important targets for the immune system. The notation HhNn is used to refer to the subtype comprising the hth discovered HA protein and the nth discovered NA protein. There are currently two subtypes circulating in humans: H1N1 and H3N2. Subtypes are further divided into strains; each genetically distinct virus isolate is usually considered to be a separate strain.