In ducks, for example, virus replication occurs in the gut epithelium before being shed in high concentration in faeces. Other creatures can then be infected, from aquatic mammals to terrestrial animals. The domestic pig is able to harbour both avian and human strains of the virus, creating a potential site for genetic shift. Infection and replication In humans, flu is a respiratory virus. Inhaled virions are trapped in respiratory tract mucous, where an initial defence mechanism reduces the viral onslaught significantly.
Immunoglobulin attack is followed by phagocytosis and mucous expectoration.
Haemagglutinin on the remaining virions reacts with sialic acid receptors on the respiratory endothelial cells. This allows the virus to invade the ciliated endothelium cell. Division and replication occur within four to six hours. Release of virions from the cell surface requires the activity of the virions' surface neuraminidase. Once released, local and distant infection occurs.