THURSDAY, Aug. 5 (HealthDay News) -- The 2009 H1N1 swine flu virus used a new biochemical trick to hijack host cells, a feat that triggered the recent pandemic, according to an international team of scientists.

"We have found why the pandemic H1N1 virus replicated so well in humans," Yoshihiro Kawaoka, a leading influenza expert and a professor of pathobiological sciences at the University of Wisconsin-Madison's School of Veterinary Medicine, said in a university news release.

The H1N1 virus is a combination of four different avian and swine flu viruses that emerged over the past 90 years. It also includes genetic residue of the 1918 pandemic virus that killed as many as 20 million people, Kawaoka explained.

A typical flu virus requires the presence of two amino acids -- lysine and asparagines -- in specific sites on a key avian protein in order to jump from an animal and replicate efficiently in human cells.

But Kawaoka and colleagues found that the lysine amino acid is located in a completely different location on the avian protein in the H1N1 virus. This is what gives the virus the ability to adapt to and co-opt human cells.

The study is published in the Aug. 5 issue of PLoS Pathogens.

"This pandemic H1N1 has this mutation and is why it can replicate so well in humans. This gives us another marker to help predict the possibility of future flu pandemics," Kawaoka said.

As of July 25, 2010, the pandemic virus had caused more than 18,398 deaths worldwide, according to the World Health Organization.

More information

The U.S. Food and Drug Administration has more about the 2009 H1N1 swine flu.