Influenza virus can live for decades and perhaps even longer in frozen lakes and might be picked up and carried by birds to reinfect animals and people, researchers reported.

Such frozen viruses could potentially become the source of new epidemics that sicken and kill generations after they were last seen, the researchers report in the Journal of Virology.

"We've found viral RNA in the ice in Siberia, and it's along the major flight paths of migrating waterfowl," said Dr. Scott Rogers of Bowling Green State University in Ohio.

"The lakes are along the migratory flight paths of birds flying into Asia, North America, Europe, and Africa," the researchers wrote.

Migrating birds are blamed, in part, for the spread of H5N1 avian influenza, which has killed or forced the culling of more than 200 million birds globally.

Since January, H5N1 has spread out of Asia, across Europe and into Africa. Now more than 50 countries have battled the virus, which has infected 258 people and killed 153 since 2003.

Experts fear it could mutate into a form that easily infects people and causes a pandemic. There were three such pandemics in the last century and one, the 1918-1919 pandemic, killed anywhere from 40 million to 100 million people.

It was caused by a virus called H1N1, a descendant of which still circulates and causes illness today.

But the original form was only recently studied and was recovered from the still-frozen body of a victim from Alaska.

Were that strain of H1N1 to circulate today, it could cause another serious pandemic because no one alive now has immunity to it, Rogers said. The original H1N1 appears to have passed fairly directly from birds to people.

Rogers noted that World Health Organization and other experts try to predict every year which strains of flu virus will be circulating, and they advise companies to formulate the next year's flu vaccine accordingly.

"Sometimes they're wrong," he said. "We thought that by looking at what's melting and what birds are picking up," better guesses for the next year might be possible.

Rogers and colleagues at the Russian Academy of Sciences sampled three lakes in northeast Siberia in 2001 and 2002. They found an H1 strain that circulated from 1933 to 1938 and again in the 1960s in the lake that had attracted the most geese.

"These certain strains come back from time to time," Rogers said.

"The data suggest that influenza A virus deposited as the birds begin their autumn migration can be preserved in lake ice. As birds return in the spring, the ice melts, releasing the viruses," the researchers wrote.

"Above the Arctic Circle, the cycles of entrapment in the ice and release by melting can be variable in length, because some ice persists for several years, decades, or longer."

Rogers said his team now wants to study lakes in Greenland and Canada.