The H2N2 and H3N2 pandemic flu strains both contained genes from avian influenza viruses. The new subtypes arose in pigs coinfected with avian and human viruses and were soon transferred to humans. Swine were considered the original "intermediate host" for influenza because they supported reassortment of divergent subtypes. However, other hosts appear capable of similar coinfection (such as many poultry species), and direct transmission of avian viruses to humans is possible. H1N1, associated with the 1918 flu pandemic, may have been transmitted directly from birds to humans.