Tests in monkeys, mice, and ferrets show that swine flu thrives in greater numbers all over the respiratory system, including the lungs, instead of staying in the nose and throat like seasonal flu.

"I'm very concerned because clearly the [swine flu] virus is different from seasonal influenza," said study lead author Yoshishiro Kawaoka. "It's a lot more severe."

The study by Kawaoka, a flu researcher at the University of University of Wisconsin, was released yesterday and will be published in Nature.

W. Paul Glezen, a flu epidemiologist at the Baylor College of Medicine in Houston who was not part of the study, said he agreed with Kawaoka that swine flu "appears to be more virulent than the seasonal" flu.

A separate study published yesterday suggests that the culprit in history's deadliest flu pandemic, in 1918, might not have made a sudden, direct jump from birds to people, as many scientists believe.

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. That argues for better surveillance of percolating flu strains, not just in the long term but right now. "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.

The study, in the Proceedings of the National Academy of Sciences, provides the first evidence that the 1918 pandemic - like the next two, in 1957 and 1968 - evolved from a series of reassortments, not a sudden jump.