Health authorities are investigating new human cases of bird flu in Vietnam. They say the cases appear to be isolated, but they are watching closely for possible signs of a wider outbreak. World Health Organization officials say a team of experts is in southern Vietnam, trying to determine how an 18-year-old girl may have contracted avian influenza, also known as "bird flu." If confirmed, hers would be Vietnam's fourth case of bird flu in humans in recent weeks. This week, Vietnamese officials reported two new deaths from the disease: a six-year-old boy who died December 30, and a nine-year-old boy who died Tuesday. A 16-year-old girl is in the hospital in serious condition with the disease. A highly contagious form of bird flu broke out in eight Asian countries last year. It was confined mostly to poultry flocks, but it also killed 22 people in Vietnam and 12 in Thailand - the only two countries in the past year to have reported human deaths from the disease. Strains of the avian flu virus are commonly transmitted among poultry. In Hong Kong in 1997, the WHO documented the first cases of poultry-to-human transmission of the potentially lethal H5N1 strain of the virus. Six of 18 people who contracted the disease then died. Health authorities fear the virus may change into a form that is easily transmissible between humans. They say that could spark a major pandemic, putting millions around the world at risk. Dr. Hans Troedsson represents the WHO in Vietnam. He says there are no signs of a wider outbreak of bird flu there at this point - but warns that an outbreak would create a difficult burden on top of the recent Asian earthquake and tsunami. "In this situation now, when we already have such a problem here in Asia with this earthquake and the aftermath of this earthquake, it will of course put a lot of strains on both individual countries like Vietnam as well as the international community," said Dr. Troedsson. Dr. Troedsson says bird-flu risk factors are likely to increase in the coming months as Asian countries prepare to celebrate Lunar New Year, a time when chicken is traditionally served. He says increased transportation of poultry and large indoor gatherings for the holiday give the virus more chances to spread. In addition, the virus is known to thrive under cold weather conditions. Hong Kong is raising its level of bird-flu preparedness in response to the new cases in Vietnam. Besides conducting routine screenings of arriving passengers, Hong Kong authorities are warning hospitals to be extra vigilant for patients who have returned from Vietnam or Thailand.