Influenza A(H1N1) virus Influenza A virus subtype H1N1, also known as A(H1N1), is a subtype of influenzavirus A and the most common cause of influenza (flu) in humans. Some strains of H1N1 are endemic in humans, including the strain(s) responsible for the 1918 flu pandemic which killed 50–100 million people worldwide. Less virulent H1N1 strains still exist in the wild today, worldwide, causing a small fraction of all influenza-like illness and a large fraction of all seasonal influenza. H1N1 strains caused roughly half of all flu infections in 2006. Other strains of H1N1 are endemic in pigs and in birds.
In March and April 2009, hundreds of laboratory-confirmed infections and a number of deaths were caused by an outbreak of a new strain of H1N1.
Influenza A virus strains are categorized according to two proteins found on the surface of the virus: hemagglutinin (H) and neuraminidase (N). All influenza A viruses contain hemagglutinin and neuraminidase, but the structure of these proteins differs from strain to strain due to rapid genetic mutation in the viral genome. Influenza A virus strains are assigned an H number and an N number based on which forms of these two proteins the strain contains. There are 16 H and 9 N subtypes known in birds, but only H 1, 2 and 3, and N 1 and 2 are commonly found in humans. Influenza A(H1N1) Current situation
Daily situation updates and maps
As of 06:00 GMT, 15 May 2009, 34 countries have officially reported 7520 cases of influenza A(H1N1) infection.
Assessing the severity of an influenza pandemic
11 May 2009 -- The major determinant of the severity of an influenza pandemic is the inherent virulence of the virus. But many other factors influence the overall severity of a pandemic’s impact.
Timeline of influenza A(H1N1) cases
This interactive world map shows the spread of new influenza A(H1N1) cases as they were reported to WHO beginning 26 April 2009. These numbers relate to laboratory confirmed cases and deaths.
GUIDANCE DOCUMENTS
For individuals
For communities
For national authorities
For health professionals
Complete list of guidance documents STATEMENTS
Sharing of influenza viruses, access to vaccines and other benefits
15 May 2009
World is better prepared for influenza pandemic
8 May 2008
Joint FAO/WHO/OIE statement on the safety of pork
7 May 2009
Director-General's statement to the UN General Assembly
4 May 2009 FREQUENTLY ASKED QUESTIONS