The influenza A virus is known to cause influenza in birds and some mammals, like humans. Different subtypes of the influenza A virus have been isolated from wild birds. Some subtypes of the influenza A virus can cause severe symptoms both in domestic poultry and (rarely) in humans and can even lead to death. Occasionally, viruses can be transmitted from wild birds to domestic animals, like chickens or pigs. This may give rise to human influenza <ref name="Arti2">Transmission of Avian Influenza A Viruses Between Animals and People, CDC, 2015, https://www.cdc.gov/flu/avianflu/virus-transmission.htm</ref>.

Influenza A viruses are negative-sense, single-stranded RNA viruses. Different subtypes of influenza A exist, these are characterized by proteins on the outermost membrane of the virus, called hemagglutinin (H or HA) and neuraminidase (N or NA). H and N are the antigens of the virus, and play an important role in the interaction between the host’s immunological response and the virus. Recently, researchers have reported the discovery of an antibody which is generally effective against all types of the influenza A virus<ref name="Arti3">Super antibody' fights off flu, BBC, 2011,James Gallagher, https://www.bbc.com/news/health-14324901</ref>.

The subtype which will be used in SensUs 2021 is H1N1. Historically, H1N1 has been responsible for most deaths due to influenza. It is a popular influenza strain for research purposes <ref name="Arti4">Prevalent Eurasian avian-like H1N1 swine influenza virus with 2009 pandemic viral genes facilitating human infection, Research Gate, 2020, Honglei Sun, Yihong Xiao, https://www.researchgate.net/publication/342555087_Prevalent_Eurasian_avian-like_H1N1_swine_influenza_virus_with_2009_pandemic_viral_genes_facilitating_human_infection</ref>