WASHINGTON (AP) – History's deadliest flu pandemic, in 1918, may not have made a sudden jump from birds to people after all.

New research says the pig played a big role as an influenza mixing bowl – a gene probe with lessons for tracking today's swine flu outbreak.

The genetic ancestor hunt shows pieces of the 1918 killer virus were quietly circulating in people and pigs up to 15 years before the pandemic erupted, researchers reported this week.

That argues for better surveillance of percolating flu strains, not just in the long term but right now. Scientists worry that the current influenza pandemic – the never-before-seen swine flu strain now crossing the globe – could become worse as it runs into other types of flu.

"We need to be vigilant for any genetic mixing of strains currently circulating in humans," lead researcher Dr. Gavin J. D. Smith of the University of Hong Kong said in an e-mail interview.

Catch two different flu viruses at once and a new one can emerge, something scientists call reassortment. Birds are the ultimate origin of influenza viruses, but because pigs can catch both bird and human flu strains, they've long been recognised as a species mixing vessel. The study, in this week's Proceedings of the National Academy of Sciences, provides the first evidence that the 1918 pandemic – like the two that followed it, in 1957 and 1968 – evolved from a series of those reassortments, not a sudden jump. The new swine flu – a part-pig, part-human, part-bird virus that's in the same influenza family, called H1N1 – also evolved gradually.

"The one thing that we do know about this 2009 H1N1 is that it likes to mate," said study co-author Dr. Robert Webster, a well-known influenza specialist from St. Jude Children's Research Hospital in Memphis.

His biggest fear isn't that the novel swine flu will mix with some regular winter flu as both types start circulating when cold weather hits – but that it might hang around long enough in places like China or Indonesia to mate with an extremely lethal bird flu that sometimes jumps from poultry to people.

"Whoa, let's hope not," said Webster, who just became an adviser to the Obama administration's flu team. "But we can't rule it out." World health authorities are closely watching the Southern Hemisphere, where flu season just began, to see how dangerous the new swine flu proves. US authorities estimate that a million people here have become ill since spring, with more than 200 deaths. While most patients recover easily, young people seem particularly vulnerable. And a related study released this week by the journal Nature warns that the swine flu spreads through the entire respiratory system, especially the lungs, more easily than its seasonal counterparts.