The estimate, released Tuesday, draws on analysis of how viruses operate, as well as their history. But it's a 'possibility' not a prediction, experts caution.
The warning is dire: Up to 90,000 "possible" deaths from a potential swine flu outbreak.
But how did the president's science advisers, who came up with the number, reach that estimate?
It is based on complicated models of how such illnesses are transmitted, how they mutate, how prepared the healthcare system is, and what's known from the patterns of past flu epidemics, such as the ones in 1918, 1957, and 1968.
It even taps the so-called "virus detectives," a branch of the Centers for Disease Control and Prevention (CDC) that tracks, in the best Sherlock Holmes manner, the threats to America's health – often traveling to remote parts of the world to study outbreaks.
The problem with dire warnings, though, is that the complacency scientists are in part trying to break may be caused by the very studies they tout – the crying wolf syndrome. The avian bird flu predictions in 2005 included estimates of millions dead. Worldwide, 282 people died.
Accordingly, after swine flu fears abated this spring, only 1 in 8 Americans is now worried "a great deal" about the virus, according to an ABC News-Washington Post poll this week.
"Scientists walk a very fine line in how much information do we give: How much do we want to make people aware versus how much do we want to downplay it?" says Sarah Bass, an expert in health-risk communication at Temple University in Philadelphia.