"We now know where the influenza-A virus comes from every year," said Pennsylvania State University Professor Edward Holmes, who led the study. "And because we now know how the virus evolves, we have a much better chance of controlling it."

Currently, there are many strains of the avian influenza virus, with viral strains H1N1, H2N2 and H3N2 also causing epidemics in humans as influenza A.

The researchers said H3N2 is the dominant strain causing most influenza infections, with lower levels of H1N1.

Holmes and his colleagues discovered new strains annually appear, infect people and then disappear. That led the scientists to conclude there must be some reservoir that generates new strains and the researchers said they believe that reservoir is located in the tropics.

"It tells us that to really understand how the influenza virus evolves on a seasonal basis, and to make the best vaccine, we need to focus our surveillance on the source population in the tropics, especially in places such as Southeast Asia," said Holmes

The study appears in the online version of the journal Nature.