The viruses then invade the cells and viral RNA takes over the cells' metabolic processes. Replication of new virus particles is initiated and continues for about 8 hours until the cells die. The replicated virus particles are then released to invade other cells nearby. Within a few hours, huge numbers of cells in the respiratory tract are dying and releasing viruses.
Influenza viruses attach to cells lining the respiratory tract to initiate infection.
The infection disrupts mucociliary clearance and other defense mechanisms. The upper respiratory tract becomes very congested and infection may spread to the lower tract to cause a viral pneumonia with capillary thrombosis, exudation and bleeding into the alveoli. Secondary invasion by bacteria from the nasopharynx may then occur and bacterial pneumonia may develop.