The 2009 pandemic had the effect of replacing prior H1N1 strains in circulation with the novel strain but not any other influenza viruses. Consequently, H1N1, H3N2, and both IBV lineages have been in circulation in seasonal form since the 2009 pandemic.In 2011, IDV was discovered in pigs in Oklahoma, USA, and cattle were later identified as the primary reservoir of IDV. In the same year, avian H7N9 was detected in China and began to cause human infections in 2013, starting in Shanghai and Anhui and remaining mostly in China. HPAI H7N9 emerged sometime in 2016 and has occasionally infected humans incidentally. Other AIVs have less commonly infected humans since the 1990s, including H5N6, H6N1, H7N2-4, H7N7, and H10N7-8, and HPAI H subtypes such as H5N1-3, H5N5-6, and H5N8 have begun to spread throughout much of the world since the 2010s. Future flu pandemics, which may be caused by an influenza virus of avian origin, are viewed as almost inevitable, and increased globalization has made it easier for novel viruses to spread, so there are continual efforts to prepare for future pandemics and improve the prevention and treatment of influenza. Etymology
The word influenza comes from the Italian word influenza, from medieval Latin influentia, originally meaning "visitation" or "influence". Terms such as influenza di freddo, meaning "influence of the cold", and influenza di stelle, meaning "influence of the stars" are attested from the 14th century. The latter referred to the diseases cause, which at the time was ascribed by some to unfavorable astrological conditions.
Each of the four species is the sole member of its own genus, and the four influenza genera comprise four of the seven genera in the family Orthomyxoviridae. They are:
Influenza A virus (IAV), genus Alphainfluenzavirus
Influenza B virus (IBV), genus Betainfluenzavirus
Influenza C virus (ICV), genus Gammainfluenzavirus
Influenza D virus (IDV), genus DeltainfluenzavirusIAV is responsible for most cases of severe illness as well as seasonal epidemics and occasional pandemics. It infects people of all ages but tends to disproportionately cause severe illness in the elderly, the very young, and those who have chronic health issues. Birds are the primary reservoir of IAV, especially aquatic birds such as ducks, geese, shorebirds, and gulls, but the virus also circulates among mammals, including pigs, horses, and marine mammals. IAV is classified into subtypes based on the viral proteins haemagglutinin (H) and neuraminidase (N). As of 2019, 18 H subtypes and 11 N subtypes have been identified. Most potential combinations have been reported in birds, but H17-18 and N10-11 have only been found in bats. Only H subtypes H1-3 and N subtypes N1-2 are known to have circulated in humans, the current IAV subtypes in circulation being H1N1 and H3N2. IAVs can be classified more specifically to also include natural host species, geographical origin, year of isolation, and strain number, such as H1N1/A/duck/Alberta/35/76.IBV mainly infects humans but has been identified in seals, horses, dogs, and pigs. IBV does not have subtypes like IAV but has two antigenically distinct lineages, termed the B/Victoria/2/1987-like and B/Yamagata/16/1988-like lineages, or simply (B/)Victoria(-like) and (B/)Yamagata(-like).