• Influenza A (H1N1) virus is the subtype of influenza A virus that was the most common cause of human influenza (flu) in 2009, and is associated with the 1918 outbreak known as the Spanish Flu.
• It is an orthomyxovirus that contains the glycoproteins haemagglutinin and neuraminidase.
• For this reason, they are described as H1N1, H1N2 etc. depending on the type of H or N antigens they express with metabolic synergy. Haemagglutinin causes red blood cells to clump together and binds the virus to the infected cell.
• Neuraminidase are a type of glycoside hydrolase enzyme which help to move the virus particles through the infected cell and assist in budding from the host cells.
• Some strains of H1N1 are endemic in humans and cause a small fraction of all influenza-like illness and a small fraction of all seasonal influenza.
• H1N1 strains caused a small percentage of all human flu infections in 2004–2005.
• Other strains of H1N1 are endemic in pigs (swine influenza) and in birds (avian influenza).
• In June 2009, the World Health Organization (WHO) declared the new strain of swine-origin H1N1 as a pandemic.
• This strain is often called swine flu by the public media.
• This novel virus spread worldwide and had caused about 17,000 deaths by the start of 2010.
• On August 10, 2010, the World Health Organization declared the H1N1 influenza pandemic over, saying worldwide flu activity had returned to typical seasonal patterns.

Source:Wikipedia