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===Humans===
{{further|Influenza}}
Avian flu viruses, both HPAI and LPAI, can infect humans who are in close, unprotected contact with infected poultry. Incidents of cross-species transmission are rare, with symptoms ranging in severity from no symptoms or mild illness, to severe disease that resulted in death.<ref>{{Cite web |last=CDC |date=2024-05-30 |title=Avian Influenza A Virus Infections in Humans |url=https://www.cdc.gov/flu/avianflu/avian-in-humans.htm |access-date=2024-06-11 |website=Centers for Disease Control and Prevention |language=en-us}}</ref><ref name=":B7">{{Cite web |date=11 June 2024 |title=Questions and Answers on Avian Influenza |url=https://ec.europa.eu/commission/presscorner/detail/en/qanda_24_3169 |access-date=2024-06-11 |website=An official website of the European Commission}}</ref> As of February, 2024 there have been very few instances of human-to-human transmission, and each outbreak has been limited to a few people.<ref>{{Cite web |date=2024-02-01 |title=Reported Human Infections with Avian Influenza A Viruses {{!}} Avian Influenza (Flu) |url=https://www.cdc.gov/flu/avianflu/reported-human-infections.htm |access-date=2024-06-11 |website=Centers for Disease Control and Prevention |language=en-us}}</ref> All subtypes of avian Influenza A have potential to cross the species barrier, with [[Influenza A virus subtype H5N1|H5N1]] and [[Influenza A virus subtype H7N9|H7N9]] considered the biggest threats.<ref>{{Cite web |title=Zoonotic influenza |url=https://www.who.int/news-room/spotlight/influenza-are-we-ready/zoonotic-influenza |access-date=2024-06-16 |website=Wordl Health Organization |language=en}}</ref><ref>{{Cite web |title=The next pandemic: H5N1 and H7N9 influenza? |url=https://www.gavi.org/vaccineswork/next-pandemic/h5n1-and-h7n9-influenza |access-date=2024-06-16 |website=Gavi, the Vaccine Alliance |language=en}}</ref>
In order to avoid infection, the general public are advised to avoid contact with sick birds or potentially contaminated material such as carcasses or feces. People working with birds, such as conservationists or poultry workers, are advised to wear appropriate personal protection equipment.<ref>{{Cite web |last= |date=2024-06-05 |title=Highly Pathogenic Avian Influenza A(H5N1) Virus in Animals: Interim Recommendations for Prevention, Monitoring, and Public Health Investigations |url=https://www.cdc.gov/bird-flu/prevention/hpai-interim-recommendations.html |access-date=2024-06-13 |website=Centers for Disease Control |language=en-us}}</ref>
The avian influenza [[Hemagglutinin (influenza)|hemagglutinin]] prefers to bind to alpha-2,3 [[sialic acid]] receptors, while the human influenza hemagglutinin prefers to bind to alpha-2,6 sialic acid receptors.<ref name= "pmid20629046">{{cite journal |vauthors= Bertram S, Glowacka I, Steffen I, Kühl A, Pöhlmann S |title=Novel insights into proteolytic cleavage of influenza virus hemagglutinin |journal=Reviews in Medical Virology |volume=20 |issue=5 |display-authors =3| pages=298–310 |date= September 2010 |pmid=20629046 |pmc=7169116 |doi=10.1002/rmv.657 |quote= The influenza virus HA binds to alpha 2–3 linked (avian viruses) or alpha 2–6 linked (human viruses) sialic acids presented by proteins or lipids on the host cell surface.}}</ref><ref name= "Shinya">{{cite journal |vauthors= Shinya K, Ebina M, Yamada S, Ono M, Kasai N, Kawaoka Y | display-authors =3| title=Avian flu: influenza virus receptors in the human airway |journal=Nature |volume=440 |issue= 7083 |pages=435–436 |date= March 2006 |pmid=16554799 |doi= 10.1038/440435a| bibcode= 2006Natur.440..435S |s2cid=9472264 }}</ref> This means that when the H5N1 strain infects humans, it will replicate in the lower respiratory tract (where alpha-2,3 sialic acid receptors are more plentiful in humans) and consequently cause [[viral pneumonia]].<ref name="Riel" /><ref name="Bennett 2021 h112">{{cite journal |last=Bennett |first=Nicholas John |title= Avian Influenza (Bird Flu): Background, Pathophysiology, Epidemiology |website= Medscape Reference |date=13 October 2021 |url=https://emedicine.medscape.com/article/2500029-overview |access-date=28 April 2024 |quote= Avian influenza is still primarily a respiratory infection but involves more of the lower airways than human influenza typically does. This is likely due to differences in the hemagglutinin protein and the types of sialic acid residues to which the protein binds. Avian viruses tend to prefer sialic acid alpha(2-3) galactose, which, in humans, is found in the terminal bronchi and alveoli. Conversely, human viruses prefer sialic acid alpha(2-6) galactose, which is found on epithelial cells in the upper respiratory tract.}}</ref> As of April 2024, there is no human-adapted form of H5N1 influenza, so all humans who have caught it so far have caught avian H5N1.▼
▲The avian influenza [[Hemagglutinin (influenza)|hemagglutinin]] prefers to bind to alpha-2,3 [[sialic acid]] receptors, while the human influenza hemagglutinin prefers to bind to alpha-2,6 sialic acid receptors.<ref name=
{{#section:Human mortality from H5N1|H5N1_WHO}}
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==Epidemiology==
{{Further|Transmission and infection of H5N1|Global spread of H5N1}}
The earliest infections of humans by H5N1 coincided with an [[epizootic]] (an epidemic in nonhumans) of H5N1 influenza in Hong Kong's poultry population in 1997. This [[panzootic]] (a disease affecting animals of many species, especially over a wide area) outbreak was stopped by the killing of the entire domestic poultry population within the territory. However, the disease has continued to spread; outbreaks were reported in Asia again in 2003. On December 21, 2009, the WHO announced a total of 447 cases which resulted in the deaths of 263.<ref name=":0">{{Cite report |url=https://www.cdc.gov/niosh/docs/2008-128/pdfs/2008-128.pdf?id=10.26616/NIOSHPUB2008128 |title=Protecting Poultry Workers from Avian Influenza (Bird Flu) |publisher=Department of Health and Human Services, Public Health Service, Centers for Disease Control and Prevention, National Institute for Occupational Safety and Health |doi=10.26616/NIOSHPUB2008128 |location=US |id=DHHS (NIOSH) Publication Number 2008–128 |year=2008 |website=cdc.gov}}</ref><ref name="who.int">{{cite web |publisher=World Health Organization (WHO) |url=https://www.who.int/influenza/human_animal_interface/EN_GIP_LatestCumulativeNumberH5N1cases.pdf |title=Cumulative Number of Confirmed Human Cases for Avian Influenza A/(H5N1) Reported to WHO, 2003–2011 |url-status=live |archive-url=https://web.archive.org/web/20111027125616/http://www.who.int/influenza/human_animal_interface/EN_GIP_LatestCumulativeNumberH5N1cases.pdf |archive-date=2011-10-27 }}</ref>
===Contagiousness===
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