World health experts have tossed out death estimates from a potential bird flu pandemic ranging from 2 million and 150 million. While the estimates vary wildly, observers say the wide range may not be entirely surprising.
After all, there is a lot of uncertainty surrounding the bird flu, including what the virus even will look like if and when it becomes capable of spreading directly between humans.
"We can't predict what a virus we've never seen will do," Marc Lipsitch, epidemiology professor at the Harvard School of Public Health, told the ABC News Internet streaming channel ABC News Now.
Current human cases of bird flu, which has killed dozens of people in Asia, appear to have spread solely from birds to humans.
"In order to become a pandemic, it will have to change" to a human-to-human strain, Lipsitch said. "And we don't know what that changed strain will do."
Just as varied, apparently, are the predictions on deaths.
Today, the United Nations' World Health Organization suggested between 2 million and 7.4 million people could die from a global influenza pandemic.
On the other hand, David Nabarro, the recently-appointed senior United Nations system coordinator for avian and human influenza, told the media Thursday that deaths from bird flu could reach into the hundreds of millions.
"I'm not, at the moment, at liberty to give you a prediction on numbers," Nabarro said, "but I just want to stress, that, let's say, the range of deaths could be anything from 5 to 150 million."
A draft report of the federal government's emergency plan predicts that as many as 200 million Americans could be infected and 200,000 could die within a few months if the bird flu came to the United States. Right now, there is no vaccine to stop the flu.
Likewise, a 1999 study by experts at the U.S. Centers for Disease Control and Prevention used a mathematical model to project that U.S. deaths from the next, unspecified influenza pandemic -- not specifically bird flu -- could reach into the hundreds of thousands.
"Using death rates, hospitalization data, and outpatient visits, we estimated 89,000 to 207,000 deaths; 314,000 to 734,000 hospitalizations; 18 to 42 million outpatient visits; and 20 to 47 million additional illnesses," the study said.
Still, Bill Hall, a spokesman for the U.S. Department of Health and Human Services, said the U.S. government does not make official predictions on illness and death rates from a flu pandemic, and said the CDC study "doesn't represent the official government estimate."
"There's no way to speculate on a number," Hall said. "We just don't do that."
The government does, however, point to the high death tolls from past influenza pandemics to illustrate potential danger, Hall said.
What's more, history suggests worldwide death estimates as high as 150 million may not be unreasonable. Adjusting for increases in the world's population, Lipsitch said, such a figure would roughly match the toll of the 20th century's worst pandemic, the "Spanish flu" of 1918, which killed perhaps 50 million people worldwide, including more than 500,000 in the United States.
On the other hand, history might point to lower estimates as well, since the 20th century's two other influenza pandemics, the "Asian flu" of 1957-58 and the "Hong Kong flu" of 1968-69, killed fewer people as percentages of the world population. According to the CDC's Web site, the Asian flu killed about 70,000 people in the United States, and the Hong Kong flu killed about 34,000.
"If we were sure it was going to be like history, we would still be unsure," Lipsitch told ABCNEWS.com. "But we can't be sure it will be like historical examples."
Beyond the issue of whether historical precedents will be repeated, there's even more uncertainty.
Marvin J. Bittner, an associate professor at the Creighton University School of Medicine, suggested it may be wrong to predict the lethality of the bird flu solely on the fact that about half of known human cases in Asia have resulted in death.
"The percentage of people who died from bird flu seems to be high when you look at the data from Asia," Bittner said. "However, this could be misleading. Sometimes, when you study a disease, the worst cases are easiest to recognize. … Later on, you learn to recognize milder cases, and so the disease looks less severe.
"You may have had some people who didn't feel sick, didn't feel sick enough to see the doctor, or didn't look sick enough to have tests for bird flu -- yet they had bird flu," Bittner added. "I don't know if this happened, but it's a theoretical possibility."
Regardless of death estimates, Lipsitch said Americans and their government officials should not take the threat from the bird flu lightly.
"Even a mild pandemic kills, in this country, some tens of thousands, and in the world, a lot more than that," Lipsitch said. "And it does so in a wide range of age groups. It's not like a normal flu that kills primarily the elderly. … We obviously need to plan for the worst case, but also know that in a best case our plans could save a lot of lives."