Influenza A virus subtype H1N1, also known as A(H1N1),
is a subtype of influenza virus 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.