Entry into the cell is facilitated by the binding of the HA spike to mucoproteins of the mucus membranes. After binding, the virus is engulfed via endocytosis. The HA monomers are cleaved at their bases by a trypsin-like enzyme. This cleavage causes a conformational change in the HA spike, that causes the virus to be taken into the actual cytoplasm. Once inside the cytoplasm, the virus begins replication.
Influenza is spread primarily by aerosols , but occasionally through a fomite .
The primary infection involves the ciliated epithelial cells of the upper respiratory tract. These cells begin necrosis that results in the usual acute respiratory infection symptoms including fever, chills, muscular aching, and headache. The incubation period is one to four days long, with an infection period lasting three to seven days. Mortality from a primary influenza infection is extremely rare. Ninety percent of all flu deaths are in the sixty-five and older age bracket, with the other ten percent consisting of persons with host factors such as an immunodeficency.