Morphology: The envelope is traversed by several hundred spikes.

The single stranded RNA is negative sense. It has a different gene on each segment. The 8 segments are held together by the helical capsid comprised of nucleoprotein.

Each gene codes for one protein: haemagglutinin (H) spike, neuraminidase (N) spike, matrix (which lines the inside of the envelope and is like scaffolding), nucleoprotein, 3 viral polymerases and a large non-structural protein.

H enables the virus to attach to respiratory epithelial cells within seconds via sialic acid on the host cell. H also attaches to red blood cells in-vitro, hence its name. Such haemagglutination is blocked when virus is pretreated with antibody (Haemagglutination inhibition (HI) test).

H is formed from a precursor protein called Ho after cleavage by host cell proteases. This cleavage normally only occurs in respiratory or gut epithelial cells. If the virus is virulent and has more basic amino acids at its H cleavage site then the proteases in other cell eg neurones can cleave Ho and thereby allow virus to grow in the brain eg seal influenza.

N is a sialidase enzyme which which prevents new virus simply reattaching to the same host cell and allows it to move to new cells.

Cultivation and cytopathic effect:

Influenza viruses are routinely grown to high titre in the allantoic cavity of 10-day-old fertile hens eggs or more rarely in kidney cells for vaccines and diagnosis. Virus is detected by its ability to haemmagglutinate red blood cells.