By ALEXANDER G. HIGGINS GENEVA Mar 6, 2006 (AP)â 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 more than $10 billion in its spread through poultry around the world, the World Health Organization said Monday. 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 Chan, citing U.N. Food and Agriculture Organization estimates of the toll on farmers. "Concern has mounted progressively, and events in recent weeks justify that concern," Chan, who is leading WHO's efforts against bird flu, told a meeting in Geneva on global efforts to prepare for the possibility of the flu mutating into a form easily transmitted among humans.