Type B and C viruses are specific to humans, while influenza A virus can mutate and acquire the ability to infect both humans and animal hosts (birds or mammals).

The virus is transmitted mainly through aerosols, but the contamination can also be transmitted from person to person through direct contact or through surfaces and objects contaminated with secretions.
The infected person is contagious 2 days before the first symptoms appear and 5 days after the onset of the disease.

Influenza syndromes are the clinical manifestations of viral infection with influenza virus at
the level of the upper respiratory epithelium, as a result of which the body acquires temporary immunity through secretion of specific antibodies.

Multiplication of the pathogen in the upper respiratory tract is followed by transient viremia that may develop in some cases (children, the elderly, immunocompromised persons) with the spread of the infectious process and the occurrence of influenza-type pneumonia due to a secondary bacterial superinfection with S.aureus, H.influenzae or haemolytic streptococci.