Many Scientists Fear Bird Flu Cases Exceed Data

Then last month, he learned of an unsettling study. Japanese researchers retested samples from 30 Vietnamese patients whose lab tests showed no signs of the disease. They discovered that seven had actually been infected.

"We are especially worried, because it may mean we missed some patients," said Tran, the hospital's deputy director.

Tran is part of a growing consensus that the extent of human bird flu infection in Southeast Asia may have been significantly underestimated.

In the last few months, scientists have begun to believe that the inaccuracy of laboratory tests, the wide variation of symptoms and the inability of public health agencies to combat the disease may have created the erroneous perception that bird flu is still rare among humans.

The number of infections is key. The more there are, the greater the chance the virus will mutate into a form that can easily be passed between people, who would have little immunity to the new disease. Scientists believe that nearly all infections so far have been caused by contact with sick or dead poultry.

Officially, the tally doesn't sound alarming. The virus has killed 14 people since December and 46 over the last 15 months. All but one of them were from Vietnam and Thailand. Altogether, there are 69 lab-confirmed cases, according to the World Health Organization.

Yet doctors and public health officials point to a glaring oddity in the statistics that underscores the belief that the case count is too low to be true.

Vietnam and Thailand have reported the overwhelming majority of recent cases. Yet Laos, which is sandwiched between the countries, has reported no cases among people or birds this year. Cambodia, which is also flanked by Vietnam and Thailand, has confirmed only a single human case.

"People are not trying to cover it up, but given how widespread the infection is in poultry in Southeast Asia, it's hard to believe people have gotten ill in only [three] countries," said Jeremy Farrar, a University of Oxford flu researcher at the Hospital for Tropical Diseases.

Philippe Buchy, head of virology at the Pasteur Institute, the only testing center in Cambodia, said, "The best way not to find something is not to look for it."