H1N1 is a subtype of the species Influenza A virus. 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 over about a year in 1918 and 1919 . A different variant exists in pig populations. Controversy arose in October 2005, after the H1N1 genome was published in the journal Science. Many fear that this information could be used for bioterrorism.
"When he compared the 1918 virus with today's human flu viruses, Dr. Taubenberger noticed that it had alterations in just 25 to 30 of the virus's 4,400 amino acids. Those few changes turned a bird virus into a killer that could spread from person to person."
Low pathogenic H1N1 strains still exist in the wild today, causing roughly half of all flu infections in 2006.
