But Offit said he backed all preparedness efforts because he expected another pandemic from an H1, H2 or H3, the subtypes responsible for six previous epidemics, including the catastrophic one in 1918.

"What I worry is that this has been a 'boy who cried wolf' phenomenon," he said. "When the next pandemic comes, people will say, 'Yeah, yeah, we heard that last time."'

Some who were Cassandras in 2005 still are.

The fact that human cases fell slightly last year is "pretty much meaningless," argued Michael T. Osterholm, director of the Center for Infectious Disease Research and Policy at the University of Minnesota. The virus is still circulating and has evolved 10 clades and hundreds of variants.

World preparations thus far are "incremental," he said, "like sending 10 troops to a war when you need 10,000."

He noted that the H3N8 flu found in horses in the 1960s took 40 years to adapt to dogs, but that since 2004 it has spread to kennels all over the country.

The most worrisome aspect of H5N1, virtually all scientists agree, is that it persists in birds without becoming less lethal to them.

"This is the most serious bird flu virus that has ever been known," Nabarro said. "By 2007, it was in 60 countries. It must be dealt with."

Despite the culling of hundreds of millions of birds and the injection of billions of doses of poultry vaccine, the virus is out of control in some of the most populous countries — though exactly which ones are in dispute, because some are touchy about conceding that they cannot rid their flocks of it.

Vallat has named three countries where it is now endemic in local birds: Egypt, Indonesia and Nigeria.

Nabarro added Bangladesh, Vietnam and parts of China. Reports of recurrent outbreaks also persist in parts of India, Myanmar and Pakistan. Last week, villagers in India were reported to be killing and eating their flocks before government cullers, who paid less than a third of market value, could seize them.

Dr. Henry L. Niman, a biochemist in Pittsburgh whose Web site tracks mutations, argues that there is a separate reservoir in wild birds that extends across Eurasia. Late each fall, fresh outbreaks appear across Europe and down into the Middle East as geese and swans migrate from Asia toward Africa.

In December, dying birds were found in Poland and Russia, in Saudi Arabia and even in a kindergarten petting zoo in Israel.

On Jan. 8, it reached one of England's most famous swan-breeding grounds, the Abbotsbury Swannery, which has been around since the 11th century.