A fatally ill Thai girl probably spread deadly bird flu to her relatives last year in what would mark the first documented case of human-to-human transmission of the feared virus, medical investigators said on Monday.

But they said there was no evidence that the H5N1 bird flu virus, which infected 44 people in Asia and killed 32 of them last year, has found a more efficient way to infect humans - although the threat remains of an influenza epidemic powerful enough to rival those of 1918, 1957 and 1968.

“It was reassuring that no further transmission of the virus has been detected,” said a team led by Kumnuan Ungchusak at the Thai Ministry of Public Health in Nonthaburi.

The report is being released early by the New England Journal of Medicine, on Monday, to coincide with a conference given by The University of Michigan Bioterrorism Preparedness Initiative.

In a Journal editorial, Klaus Stohr of the World Health Organization said the world must put up safeguards now.

“The emergence of human cases of avian influenza A (H5N1) virus infection in Asia is an unprecedented warning and has given the world more time to prepare than anyone might have expected,” he said.

The threat of a bird flu epidemic in humans has health officials so worried that more than 120 million poultry birds were destroyed during the first quarter of last year in an effort to control its spread among animals.

There have been other instances where doctors suspected that the virus had spread between humans, but it was hard to know if the victims had been exposed to the same source of the virus.

In this case, however, the evidence suggested that an 11-year-old Thai girl transmitted the disease to her mother and aunt last year, the Ungchusak team said.

“Disease in the mother and aunt probably resulted from person-to-person transmission,” they concluded.

The girl probably acquired the virus from exposure to infected poultry. But her mother, a garment factory worker who traveled from another province to care for her daughter, had no contact with birds.

The mother spent 16 to 18 hours with her daughter in the hospital on Sept. 7 and 8, hugging and kissing her. She fell ill herself on Sept. 11, dying 12 days after her daughter.

Investigators originally thought the aunt, with whom the girl lived, had acquired the virus from the same source as the girl. But it typically takes 2 to 10 days for someone to become ill after exposure to an infected bird.

In this case, the aunt became sick 17 days after her last exposure to poultry, making it likely that she got her illness from the girl as well.

The aunt, who had handled sick chickens with plastic bags on her hands and had cared for the hospitalized girl for 12 to 13 hours, survived. No hospital workers became ill.

The fact that the virus did not spread further suggests that the virus has not adapted to efficient human spread, the Ungchusak team said, adding: “But this should not be a rationale for complacency.”