Monkeys infected with a resurrected virus that was responsible for history's deadliest epidemic have given scientists a better idea of how the 1918 Spanish flu attacked so quickly and relentlessly: by turning victims' bodies against them.

The research, which found that an over-stimulated immune system killed even as it tried to fight the flu, helps explain why many of the 50 million people who died in the epidemic were healthy young adults. Conventional flu usually claims mostly the very young and very old.

This new look at an old killer gives doctors ideas on how to fight the current bird flu if it develops the ability to spreads from human to human, as many scientists fear it will. The 1918 virus, which was reconstructed with reverse genetics, exists today only in two labs where scientists are studying it.

Scientists said they were struck by how suddenly and overwhelmingly the 1918 flu struck seven macaques that were tested in a high-level biosafety lab in Winnipeg, Canada. The virus spread faster than a normal flu bug and triggered a "storm" response in the animal's immune systems.

Their bodies' defenses went haywire, not knowing when to stop, researchers said. The lungs became inflamed and filled with blood and other fluids.

The scientists believe the virus had the same effect on humans.

"Essentially people are drowned by themselves," said University of Wisconsin virology professor Yoshihiro Kawaoka, lead author of a study published in the journal Nature.

The experiment was supposed to last 21 days, but after eight days the monkeys were so sick feverish, in pain and having difficulty breathing that ethical guidelines forced the researchers to euthanize them.

"There was some surprise that it was that nasty," University of Washington virologist and study co-author Michael Katze said. "It was the robustness of the immune system that helped victimize them."

The virus was simply overwhelming, researchers said.

"It's a very good replicating virus and therefore it can affect more of the immune system and thereby triggers what one refers to as a cytokine storm," said Peter Palese, chairman of the microbiology department at Mount Sinai School of Medicine in New York, who was not part of the study but has worked on the resurrected virus before. Cytokines transmit messages among cells in the immune system.

No other flu virus is deadly to monkeys, and the speed in its spread and the overwhelming immune system response is only similar to those in the H5N1 bird flu, Kawaoka said.

The bird flu has spread around the world intermittently, but has yet to develop the ability to transmit person-to-person. If it does, scientists believe understanding the 1918 flu may give them clues about how to protect people from it.