a type of influenza virus that can be found in birds, pigs, and humans. Strains of H1N1 can mix with other flu viruses within the host to form new strains. This rapid mutation and the ability of H1N1 to infect multiple species can make predicting flu pandemics difficult and prevent the creation of a single vaccine effective against all H1N1 strains. H1N1 is believed to have caused the 1918 Spanish flu pandemic. In 2009, a strain of H1N1 virus believed to have originated in pigs broke out in Mexico and spread to the United States, where it was declared a national emergency. The 2009 swine flu pandemic, as it was called, claimed 203,000 lives.