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
- Main article: 2009 pandemic
The 2009 Flu outbreak is a new strain of H1N1-type influenza, commonly referred to as "swine flu". This strain mutated out of other strains of swine flu and other influenza viruses.
Cases were found to be made up of genetic elements from four different flu viruses – North American swine influenza, North American avian influenza, human influenza, and a swine influenza virus typically found in Asia and Europe.