Nearly two-thirds of Americans are worried about the threat of a future smallpox attack, and 17 percent of those are "very worried," according to the Gallup poll.

The rise in fears seems to parallel the increased media coverage of the tensions between Iraq and the United States. In fact, more Americans link their fears of bioterrorism to a war with Saddam Hussein than with terrorist organizations such as al Qaeda.

Despite the potential side effects of the vaccine, 53 percent of those surveyed said they would get immunized.

Health officials have warned that one or two people out of a million who get the vaccine for the first time will die, and up to 50 people will face serious complications. Those include a scarring rash, high fever and encephalitis, a swelling of the brain.