According to this study, the phylogenetic origin of the flu virus that caused the 2009 pandemics can be traced before 1918. Around 1918, the ancestral virus, of avian origin, crossed the species boundaries and infected humans as human H1N1. The same phenomenon took place soon after in America, where the human virus infected pigs; it led to the emergence of the H1N1 swine strain, which later became known as swine flu. Genetic coding of H1N1 shows it is a combination of segments of four influenza viruses forming a novel strain: