Filed at 5:33 p.m. ET

WASHINGTON (AP) -- A bioterror attack using smallpox is an ``extremely realistic'' possibility, but vaccinating all Americans in advance isn't a good idea, one of the nation's top infectious disease experts said Monday.

How likely is a smallpox attack? ``It's a real threat, but you can't put a number on it,'' said Dr. Anthony Fauci of the National Institutes of Health.

That's why, unquestionably, the public will debate whether the smallpox vaccination that ended in the 1970s should resume, he said. So far the government opposes that, a position Fauci endorses, because the smallpox vaccine can cause such serious side effects that for every 1 million people vaccinated, one or two could die.

Still, ``since this is a very difficult issue ... there should be an open discussion, be that at a hearing or an open town meeting, so that information can be given to the American public so they can understand'' that position, Fauci told reporters Monday.

Fauci said the anthrax-by-mail attacks taught the importance of being up front with the public. Government officials made mistakes early in the anthrax outbreak by trying to reassure Americans rather than simply stating what they knew -- and didn't know -- about the risks, he said.

``The need to avoid panic is important. But denial of risk when a risk is present can undermine confidence,'' he said.

``The New Year's resolution of people in public health, myself included, is the promise not to be afraid to say that although the risk of a bioterrorist attack that will affect you or you or you is relatively small compared to other risks that you take, it is not zero.''

Fauci, head of the NIH's National Institute of Allergy and Infectious Diseases, helped handle the anthrax crisis and is considered among the top candidates to become the NIH's new director.

While the nation is better prepared now, no one can guarantee that some Americans won't die from another bioterror attack, he said.

If law enforcement doesn't prevent terrorism, ``this is tough but it's the truth: You have to rely on patterns that unfortunately will require people getting sick and perhaps dying before you know you're actually in the middle'' of a bioterror-caused disease outbreak, he said.

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On the Net:

National Institute of Allergy and Infectious Diseases: http://www.niaid.nih.gov/

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