What are the causes of influenza? How is influenza transmitted?

Flu is caused by influenza virus, which belongs to the family of Orthomyxoviridae, is a spherical or filamentous RNA virus with a diameter of 80-120nm. There are three types of influenza virus: influenza A (A type), influenza B (B type) and influenza C (C type).

Among them, type A is the most likely to mutate and can infect humans and various animals. It is the main pathogen of human influenza and often causes large, medium and small epidemics. Type B influenza virus mutates less and can infect humans, causing outbreaks or small epidemics. Type C is relatively stable and can infect humans, mostly in sporadic cases. Currently, pigs can also be infected. Influenza viruses are not heat-resistant, 100℃ for 1 minute or 56℃ for 30 minutes. They are sensitive to ultraviolet rays to commonly used disinfectants (1% formaldehyde, peracetic acid, chlorine-containing disinfectants, etc.), and can survive low-temperature drying, vacuum drying or below -20℃.

Influenza A virus is the most common and can be widespread and zoonotic, such as the bird flu that raged in Hong Kong in 1997, forcing the government to slaughter 1.5 million chickens. Influenza A viruses can be reclassified based on their structure, such as the H5N1 strain of influenza A virus (Hong Kong avian influenza virus), H3N2 (occurred in Wuhan in 1995), H1N1 (occurred in Germany in 1995), etc.

Due to irregular gene mutations, new viruses emerge. Influenza A virus mutation is a common natural phenomenon, mainly H and N. The so-called H and N refer to two proteins on the surface of influenza A viruses. H is hemagglutinin, which is like a key that helps the virus open the door of the host cell; N is neuraminidase, which can destroy cell receptors and allow the virus to spread freely in the host. The morphology of H and N influenza A viruses can have 15 H types and 9 N types, such as H1N1 and H5N1.