Bird flu could be making a possible resurgence following the discovery of a new strain of the virus that has evaded existing vaccination regimes, the United Nations has warned.
In a statement on Monday, the UN's Food and Agriculture Organisation (FAO) said there were signs that a "mutant strain" of the H5N1 virus was spreading in Asia and beyond.
According to the World Health Organisation (WHO) the virus has infected 565 people since 2003 and has killed 331. Fears of a worldwide pandemic of the virus in 2005 led to governments stockpiling anti-viral drugs.
The new strain, know as H5N1 - 2.3.2.1, has been discovered in China and Vietnam, which suspended its springtime poultry vaccination campaign this year.
The FAO's chief veterinary officer, Juan Lubroth, said the advance of the virus into countries that had previously been free from infection was related to the migratory patterns of birds.
"The general departure from the progressive decline observed in 2004-2008 could mean that there will be a flareup of H5N1 this fall and winter, with people unexpectedly finding the virus in their backyard," Lubroth said.
"The countries where H5N1 is still firmly entrenched – Bangladesh, China, Egypt, India, Indonesia and Vietnam – are likely to face the biggest problems but no country can consider itself safe.
He added: "This is no time for complacency. No one can let their guard down with H5N1."
Recently affected areas are to be found in Israel and the Palestinian Territories, Bulgaria, Romania, Nepal and Mongolia.