The H1N1 virus that caused the global pandemic last year used a biochemical trick to spread efficiently in humans, according to a new study released Thursday.

The virus caused a worldwide epidemic in 2009-2010 that sickened up to 34 million Americans and caused an estimated 6,000 U.S. deaths.

The report in Public Library of Science Pathogens by Yoshihiro Kawaoka, professor of pathobiological sciences at the University of Wisconsin School of Veterinary Medicine, says H1N1 used a different way to jump from an animal host to humans than previously discovered by scientists.

The finding of the mutation in the H1N1 virus helps explain how the virus replicated so well in humans. “This gives us another marker to help predict the possibility of future flu pandemics,” says Kawaoka, one of the world’s leading influenza experts.

The H1N1 virus is a combination of four different bird and swine flu viruses that have emerged over the past 90 years, and even includes genetic residue of the 1918 pandemic virus that killed up to 20 million people worldwide.