Influenza A virus subtype H1N1
subtype of the influenza A virus, with some strains that are swine flue, others avian flue
Influenza A virus subtype H1N1 (A/H1N1) is a subtype of influenza A virus (IAV). Some human-adapted strains of H1N1 are endemic in humans and are one cause of seasonal influenza (flu). Other strains of H1N1 are endemic in pigs (swine influenza) and in birds (avian influenza). Subtypes of IAV are defined by the combination of the antigenic hemagglutinin (H) and neuraminidase (N) proteins in the viral envelope; for example, "H1N1" designates an IAV subtype that has a type-1 H protein and a type-1 N protein.
All subtypes of IAV share a negative-sense, segmented RNA genome. Under rare circumstances, one strain of the virus can acquire genetic material through genetic reassortment from a different strain and thus evolve to acquire new characteristics, enabling it to evade host immunity and occasionally to jump from one species of host to another. Major outbreaks of H1N1 strains in humans include the 1918 Spanish flu pandemic, the 1977 Russian flu pandemic and the 2009 swine flu pandemic, all of which were caused by strains of A(H1N1) virus which are believed to have undergone genetic reassortment.
Each year, three influenza strains are chosen for inclusion in the forthcoming year's seasonal flu vaccination by the Global Influenza Surveillance and Response System of the World Health Organization (WHO). Since 1999, every annual formulation has included one strain of A/H1N1 as well as two other influenza strains – together representing strains thought most likely to cause significant human suffering in the coming season.
Swine influenza
Swine influenza (also known as swine flu or pig flu) is a respiratory disease that occurs in pigs that is caused by the Influenza A virus.
This brief starts where responsible research should: with the source description of “Influenza A virus subtype H1N1” as subtype of the influenza A virus, with some strains that are swine flue, others avian flue. Everything that follows is an evidence route, not borrowed authority.
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The subject matters to the general reference register because the source frames it as subtype of the influenza A virus, with some strains that are swine flue, others avian flue. Its deeper value depends on whether names, dates, institutions and citations support that framing.
The citation trail is more important than the brevity of the summary: it shows where individual claims can be examined in context. The source revision retrieved here is dated Sep 8, 2026. The linked authority identifier is Q161393. The Library of Congress control number is sh2009009118. 1 of 1 selected statements include explicit references; 0 carry qualifiers and 0 use preferred rank. The first chronological checks are 1918, 1977, 2009 and 1999.
The absence of detail may reflect summary conventions rather than a lack of surviving documentation. The lead is largely declarative, so disagreement and counter-evidence require a deliberate search beyond the opening account. Authority statements aid reconciliation but still require their own references, qualifiers and ranks to be checked.
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This entry incorporates text from “Influenza A virus subtype H1N1” on English Wikipedia. Contributors are listed in the page history. Text is available under the Creative Commons Attribution-ShareAlike 4.0 License. Selected authority identifiers and statements are retrieved from Wikidata under CC0; their references and qualifiers remain part of the verification path.