Type A belongs to the Orthomyxovirdae virus family is enveloped and is pleiomorphic with a size ranging from 80-120 nm.
Although there are many subtypes of the A virus, there is only one known subtype of the B and C viruses.
The HA has 16 subtypes (H1 – H16) and contains neutralizing epitopes. Antibodies against the NA are not neutralizing, and there are nine neuraminidase or ‘N’ types. All 16 subtypes have been found in ducks, gulls, or shorebirds, the natural reservoir host species of the virus.
There are two major ways in which influenza A viruses can change antigenically: via antigenic drift or shift. During antigenic drift, a variety of mutations including substitutions, deletions and insertions produce genetic variation in the surface proteins.
A second type of variation, antigenic shift, describes a major antigenic change whereby a virus with new H is introduced into the human population.
Avian flu type A viruses