A Vietnamese couple who ran a chicken farm and their three daughters have tested positive for bird flu (search), in the largest group of infections reported in the country at one time, health officials said Tuesday.

The cluster of infections raises the specter that the virus might have been spread from person to person.

Health experts fear the bird flu virus could mutate into a form that is easily transmitted between humans, triggering a pandemic that could kill millions. However, there has been no evidence of that so far, with most cases traced to contact with sick birds.

Doctors said they believe the family became infected through direct contact with infected poultry.

"I'm not very worried (about human to human transmission) because they all got sick at the same time so it's more likely that they got infected from the same source," said Nguyen Tran Hien, director of the Institute of Hygiene and Epidemiology.

Initial tests showed the family tested positive for the potent H5N1 strain of bird flu, but the results of final tests are still pending, Hien said.

Overall, 48 people in the region have been killed by the virus, which emerged on Asian poultry farms in December 2003. The disease has killed two people from Cambodia (search) and 12 in Thailand. The remainder died in Vietnam (search), including 14 in the latest wave that began in late December 2004.

The family, admitted to a hospital on March 22 with flu symptoms, raised more than 400 chickens, Haiphong Health Department director Nguyen Van Vy said. About half of the poultry began dying at the beginning of the month and the family then ate some of the chickens, he said.

Still, the possibility of human-to-human transmission remains a major concern since Vietnam has reported at least five separate family clusters which became infected since 2003, four of them from the northern province of Thai Binh. In two of those instances, the infected family members remained healthy.

Ngo Viet Hung, head of the infectious disease ward at Viet Tiep hospital, said in the latest case, the family's symptoms have largely disappeared. Family members are being kept in an isolation ward.

Hung said the family's neighbor, a 32-year-old woman, also was hospitalized on March 25 with high fever and coughing. Test results for her are still pending, he said.

Last week, a team of Vietnamese health experts was sent to Quang Binh province in central Vietnam to investigate reports of an outbreak in which scores of people displayed flu symptoms, two weeks after a 5-year-old boy had been infected with bird flu. Samples were taken from about 30 people, and results are still pending.