Avian flu continues to spread through Turkey, worrying health officials who fear the eventual outbreak of a human epidemic. A World Health Organization lab in Britain this week tested samples of the virus taken from two Turkish victims, and found a mutation that could help make the virus more transmissible from birds to people.

Also troubling is that some people seem to get infected with the often deadly H5N1 strain of the virus without developing symptoms. However, one of two boys in an Ankara hospital who tested positive for H5N1 without showing symptoms has now started to fall ill, said Dr. Guenael Rodier, head of the WHO mission to Turkey.

"When we have a child infected we are giving the virus more chance to adapt to human beings, and giving it this chance could help create conditions for the emergence of a new virus," Rodier told the Reuters news agency.

But Dr. Anthony Fauci, head of the US National Institute for Allergy and Infectious Diseases, told Reuters it wasn't clear that the mutation would lead to an epidemic. "This same mutation was identified in 2003 in Hong Kong," he said, "and yet did not take off in a way that led to greater transmissibility either from chicken to human or human to human."

The H5N1 virus has been found in wild and domesticated birds across Turkey, from Istanbul to the border with Iran. It has so far caused the deaths of at least three Turkish children and another 18 have been infected, according to the WHO.  Worldwide, H5N1 has infected 147 people and killed at least 78, almost all in China and South East Asia.

"As the new cases of human infection with the H5N1 virus in Turkey show, the situation is worsening with each passing month and the threat of an influenza pandemic is continuing to grow every day," Shigeru Omi, the WHO's regional director for the western Pacific, told a meeting of Asian countries and international organizations on bird flu in Tokyo.