"In general, people are risk averse, they want to protect themselves," said Dr. Robert Klitzman, a Columbia University Medical Center psychiatrist who has researched the AIDS and mad cow epidemics.
"You don't want to overreact or underreact, and knowing what to do is extremely hard," he told ABCNews.com. "We tend to go one way or the other. We want to avoid either one."
Today, travelers have been cancelling airline tickets, while others have been sporting face masks at airports like JFK in New York.
Nielsen Online has just released new data that shows the Internet has been buzzing furiously about risks, symptoms and other updates for information. Even people who never leave their homes are worried about swine flu.
Right now, according to Klitzman, "there's a lot of hype and fear." Next week, when the CDC gets a grasp of its spread, "things go back to normal and people relax."
Public panic is a primal reaction, according to Klitzman.
"It's an evolutionary need to respond quickly as group to a real threat," he said. "When someone says there's a hurricane or a flood coming, you need to act quickly and do something, or you are in trouble. It's in our genes."
"But sometimes, it's switched on when it's not needed," he said. "We think, better to be safe than sorry. Better to run for cover when it's not going to rain than to be laissez-faire and be flooded."
Fear can also be useful in mobilizing resources -- emotional and otherwise. The CDC noted it issued its alert to free up funding for swift detection and response.
Not to diminish the danger of flu -- typically it kills about 36,000 American deaths each year, according to CDC statistics -- but the 1918 pandemic struck before the advent of antibiotics, anti-virus drugs and sophisticated breathing machines.
"Million died, but we live in a different world now," said Klitzman. "We have thermometers now."
And some would say -- credit to Franklin D. Roosevelt -- "We have nothing to fear, but fear itself."
But even though fear is an "important motivator," until danger plays out, even public health experts can be confused about how the disease will take its course.
History shows that Americans have often overreacted to public safety threats -- from buying fall-out shelters in the 1950s to protect against nuclear Armageddon to preparing for "Y2K" at the turn of the 21st century.
In the year 2000, many predicted planes would fall from the sky, cash machines wouldn't work and medical equipment will break down. But at the stroke of midnight, no computer systems crashed.
In the jittery aftermath of the Sept. 11, 2001 terrorist attacks, Americans panicked over a series of anthrax-laced letters, haplessly sterilizing mail in their microwaves.
And when the Bush administration decided to go to war in Iraq and urged Americans to buy duct tape and sheets of plastic to protect against a retaliatory chemical attack, home improvement stores ran out of supplies in the rush.
Some medical threats caused many Americans to fear their own backyards: tick-born Lyme disease was the nadir of the 1980s and mosquito-carried West Nile disease was a scare of the 1990s.