H1N1 is a subtype of the influenza A virus. The "H" refers to the Hemagglutinin protein, and the "N" refers to the Neuraminidase protein.
H1N1 has mutated into various strains including the Spanish Flu strain (now extinct in the wild), mild human flu strains, endemic pig strains, and various strains found in birds. A variant of H1N1 was responsible for the Spanish flu pandemic that killed some 50 million to 100 million people worldwide from 1918 to 1919.
Low pathogenic H1N1 strains still exist in the wild today, causing roughly half of all flu infections in 2006. When the 1918 virus was compared with human flu viruses in 2005, it was noticed that it had alterations in just 25 to 30 of the virus's 4,400 amino acids. These changes had been enough to turn a bird virus into a version that was human-transmissible.
2009 Swine Flu outbreak
In April 2009, an outbreak of a new strain of H1N1 killed over one-hundred and fifty in Mexico, and was believed to have infected more than 1500 individuals worldwide as of April 26, 2009. This strain of H1N1 had combined with two strains of swine flu and a strain of avian flu.