Geneva ? 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.

Chan told more than 30 experts in Geneva that the agency’s top priority was to keep the deadly H5N1 strain of bird flu from mutating.

“Should this effort fail, we want to ensure that measures are in place to mitigate the high levels of morbidity, mortality and social and economic disruption that a pandemic can bring to this world,” she said.

WHO says 175 people are confirmed to have caught bird flu, and 95 of them have died.

“No one can say when this will end,” Chan said.

Global influenza pandemics – as opposed to annual recurrences of seasonal flu – tend to strike periodically. In the 20th century, there were pandemics in 1918, 1957 and 1968.

WHO said bird flu could potentially cause more deaths than those from the global flu pandemics. Because the H5N1 virus is airborne, it is easier to transmit and much more contagious than HIV/AIDS, WHO officials said.

Dr. Mike Ryan, director of epidemic and pandemic alert and response at WHO, said, “We truly feel that this present threat and any other threat like it is likely to stretch our global systems to the point of collapse.”

This is the first time world health authorities have tried to stop a global influenza pandemic before it begins. Chan referred to the spread of severe acute respiratory syndrome, or SARS, as evidence of “how much the world has changed.”

SARS infected 8,000 people, killing 800 of them.

“In a globalized economy, with high volume of international travel, vulnerability to new disease threats is universal,” she said. “It is the same for the rich and for the poor.”

WHO spokeswoman Maria Cheng said experts hope to isolate areas where there is a bird flu outbreak and establish agreements allowing international health authorities to respond quickly, testing viruses and implementing containment measures.

Public health measures to quarantine areas, isolate people or help give antiviral medicine to those infected with bird flu also are on the agenda of the meeting, which ends Wednesday.

Even if a pandemic cannot be stopped, WHO says such measures can buy time for health authorities to improve their response strategies and stave off the disease until a pandemic vaccine can be produced.