THE lethal strain of bird flu poses a greater challenge to the world than any infectious disease, including AIDS, and has cost 300 million farmers over $10 billion in its spread through poultry around the world, the World Health Organisation said yesterday. Scientists also are increasingly worried that the H5N1 strain could mutate into a form easily passed between humans, triggering a global pandemic. It already is unprecedented as an animal illness in its rapid expansion. Since February, the virus has spread to birds in 17 countries in Africa, Asia, Europe and the Middle East, said the WHO's Dr Margaret...