These are normal cilia of the airways viewed by scanning electronic microscopy. Goblet cells can be seen between the ciliated epithelial cells.

Bacteria that have entered the airways are removed by brisk ciliary motion.

A cross-section of the respiratory tract tissue.

Covered by a mucous layer, we can see ciliated epithelial cells and goblet cells.

The lamina propia mucosae below them are rich in connective fibers such as collagen and contain blood vessels. Even if the bacteria get into the airways, neutrophils that have migrated from the blood vessels encapsulate and remove them.

However, when infected by the highly tissue-invasive influenza virus, the cilia are desquamated and the mucosal epithelia are destroyed. Tissues without a barrier are destroyed is vulnerable to bacterial invasion.

When bacteria in an influenza patient cause secondary infection, this can trigger pneumonia and sometimes even result in death. Also, a secondary infection by Streptococcus pneumoniae can easily become very serious.