Experts with the World Health Organization are urging countries to step up surveillance of bird flu outbreaks in poultry. The announcement follows Pakistan's first human bird flu fatality and Burma's first reported human case of bird flu. As Naomi Martig reports from VOA's Asia News Center in Hong Kong, concern is also growing that as colder weather sweeps through much of the northern hemisphere, so will cases of the potentially deadly virus.

The World Health Organization says the bird flu virus is likely to become more prevalent in the coming months, because just as people are prone to sickness in cold weather, so are birds.

Malik Peiris teaches microbiology at Hong Kong University. He was part of a team that in 2003 provided the first genetic map of the bird flu virus and its mutations. He says more temperate parts of the world will see increased transmission of the virus.

"And the reason for this may be the fact that the virus seems to survive for a long period of time in the cooler temperature, especially when it is moist and wet," Peiris said.

Bird flu has killed more than 200 people, mostly in Asia, since 2003. The two countries with the greatest number of human cases are Indonesia and Vietnam.

Hans Troedssan, spokesman for the World Health Organization's Beijing office, says the WHO is urging health workers to be alert, including those in countries where there have been few cases of bird flu.

"There is risk which we have seen in some countries, the risk of complacency particularly among the public and thinking that there is no danger anymore," Troedssan said.

Japan, Malaysia and Vietnam are among the countries that have been praised for efficiently dealing with bird flu outbreaks in poultry. But Troedssan warns that complacency is greatest in countries that have had the most success containing the virus.

The majority of people infected by bird flu contracted the virus from sick poultry. But there have been cases of human transmission. Peiris says experts are concerned the virus could mutate into a form easily passed between people.

"The most worrying thing is that if the virus does adapt to human transmission and causes a pandemic, the severity of human disease could be catastrophic," Peiris said. "It could be much worse even than the 1918 pandemic. And I think that is why we have to take it seriously."

The World Health Organization has sent medical teams to Pakistan to investigate the possibility of human-to-human transmission in the country's first human cases of bird flu. At least five of the eight people infected are from the same family.