That's not good news for the spread of bird flu, which experts fear could mutate and be transmitted easily among people.

There are 1,407 pathogens - viruses, bacteria, parasites, protozoa and fungi - that can infect humans, said Mark Woolhouse of the University of Edinburgh in Scotland. Of those, 58 percent come from animals. Scientists consider 177 of the pathogens to be ''emerging'' or ''re-emerging.'' Most won't cause pandemics.

Experts fear bird flu could prove an exception. Recent advances in the worldwide march of the H5N1 strain have rekindled fears of a pandemic.

Controlling bird flu will require renewed focus on the animal world, including the chickens, ducks and other poultry that have been sacrificed by the tens of millions to stem the progress of the virus, experts said at a news conference late Sunday at the annual meeting of the American Association for the Advancement of Science.

''The strategy has to be looking at how to contain it in the animal world, because once you get into the human side, you're dealing with vaccines and antiretrovirals, which is a whole new realm,'' said Nina Marano, a veterinarian and public health expert with the National Center for Infectious Diseases.

Bird flu has killed at least 91 people - most of them in Asia - since 2003, according to the World Health Organization. It appears to kill about half the people it infects. However, should it mutate so it can pass from human to human, it likely will grow far less deadly, said Stanley Lemon, of the University of Texas Medical Branch at Galveston.