The latest spate of deaths from bird flu in Vietnam is raising fears that the disease is re-emerging after last year's outbreak in 10 Asian countries. This time, however, there is a new, more ominous concern: A recent medical study reports the first case of bird flu transmission among humans.

A report in The New England Journal of Medicine documents a probable case of person-to-person transmission of the avian flu virus in Thailand. Previous cases were traced to contact with infected poultry. In this instance, investigators from the Thai Health Ministry and the U.S. Centers for Disease Control say an 11-year-old girl died after exposure to a sick chicken, but her mother and aunt, who had no such bird exposure, also became ill, suggesting that human transmission occurred. The mother also died.

The president of the vaccine division of the big U.S. pharmaceutical company Merck, Adel Mahmoud, says the new report is a sign of the virus' potential to trigger a global flu outbreak. "That is just a very, very serious warning sign that viruses are recombining, moving from avian to animals to humans and then being transmitted within the human population," he said.

Continuing spread of bird flu between people is the worst fear of public health experts. The World Health Organization's representative in Hanoi, Hans Troedsson, says this would mean that the virus has genetically altered to a form much more easily transmitted. "The major concern we have is, of course, that the virus will change or alter, but we have no indication of that yet," he said. "The other concern we have would be a dramatic increase in the number of cases even if the virus has not changed, because that would put a lot of additional pressure on the existing health care services."