When advising countries on how to prepare for the next pandemic, WHO says it uses the moderate scenario because that's what's most likely to happen. It doesn't mean that a more exaggerated scenario cannot happen, only that it is less likely.

The UN health agency was fielding inquiries from both the media and the general public after Dr. David Nabarro -- a senior WHO official named Thursday as the new UN coordinator for avian and human influenza -- cited the number during a press conference at the UN's New York headquarters.

WHO's flu spokesman at the agency's Geneva headquarters made a surprise appearance Friday at the UN regular media briefing in an effort to put Nabarro's comments in context. While he did not say the 150 million prediction was wrong, or even implausible, he reiterated that WHO considers a maximum death toll of 7.4 million a more reasoned forecast.

'Pick Almost Any Number'

Scientists have made all sorts of predictions, ranging from less than 2 million to 360 million. Others have quoted 150 million.

"We're not going to know how lethal the next pandemic is going to be until the pandemic begins," said WHO influenza spokesman Dick Thompson. "You could pick almost any number," until then, he said, adding that WHO "can't be dragged into further scaremongering."

Experts agree that there will certainly be another flu pandemic -- a new human flu strain that goes global. However, it is unknown when or how bad that global epidemic will be. It is also unknown whether the H5N1 bird flu strain circulating in Asian poultry now will be the origin of the next pandemic, but experts are tracking it just in case and governments across the world are preparing themselves for such a possibility.

Two unknown factors will have a major influence on how many people will die from the next flu pandemic, experts say. One is the attack rate, or the proportion of the population that become infected. The other is the death rate, or the proportion of the sick who die.

Normal seasonal flu viruses have an attack rate of between 5 percent and 20 percent, but a death rate of less than 1 percent. Between 250,000 and 500,00 die from flu every year, according to WHO.

Most Likely Scenario

Based on evidence from the three pandemics that occurred during the 20th Century, scientists have determined that pandemic flu strains tend to infect between 25 percent and 35 percent of the population.

The worst death rate was seen in the 1918 pandemic, known as the Spanish flu pandemic. That killed 2.6 percent of those who got sick, killing a total of about 40 million people.

The other two pandemics were gentler. The 1957 one killed 2 million people and the most recent one, in 1968, killed 1 million people.

Forecasts that change the assumed attack rate or the death rate will yield different predictions. Other assumptions, such as whether or not anti-flu drugs will work against the virus, would also change the figures.

WHO said Friday that it considers the most likely scenario to be a death toll of between 2 million and 7.4 million people.

"That's because we looked at what happened in the 1918 pandemic. That caused the greatest number of deaths ever recorded from an infectious disease in a single year, by far. More than the black plague, more than any other infectious disease," said Thompson. "That was an extreme of an already rare event. Pandemics normally have more modest death rates."

How to Prepare

When the UN health agency advises countries on how to prepare for the next pandemic, it uses the moderate scenario because that's what's most likely to happen. It doesn't mean that a more exaggerated scenario cannot happen, only that it is less likely, Thompson said.

"There is a limited amount of public health money available to countries and we have to give them the best guidance on how to spend that money," he said.

The H5N1 strain of bird flu has swept through poultry populations in large swathes of Asia since 2003, jumping to humans and killing at least 65 people -- more than 40 of them in Vietnam -- and resulting in the deaths of tens of millions of birds.

Most human cases have been linked to contact with sick birds. But WHO has warned that the virus could mutate into a form that spreads easily among humans -- changing it from a bird virus to a human pandemic flu strain.