he medical experts were faced with a tough choice. They had reason to believe that an infectious disease might strike Americans, killing many thousands. An epidemic could be prevented with a vaccine, but almost no one was immunized. Should the nation stockpile vaccine, wait for the illness to strike and then start immunizing? Or should there be a crash campaign to vaccinate everyone in America?

In the end, the decision was made by the president — Gerald R. Ford, in March 1976. The feared disease was swine flu, a deadly strain that experts thought might be the same one that had killed half a million Americans in 1918. Mr. Ford decided on the crash program.

The decision backfired. Not only did the epidemic fail to materialize, but many people also developed an exaggerated fear of the vaccine itself. The vaccination campaign was abandoned midstream, and the director of the Centers for Disease Control was summarily fired amid scathing criticism of a public health program gone awry.

Now, as medical leaders call for a national debate over how to respond to the remote possibility that terrorists would spread smallpox in the United States, some say the swine flu story can provide valuable lessons. It showed the importance of building a national consensus for a new immunization campaign and the emotional power of the anecdote to erode a consensus — how the story of a single vaccine victim, or even one who mistakenly believed he was injured by a vaccine, can overwhelm logical arguments about risks and benefits.

Flu, of course, is not smallpox. Flu vaccines are safe; the smallpox vaccine is risky. Dr. Jonathan B. Tucker, author of "Scourge: The Once and Future Threat of Smallpox" (Atlantic Monthly Press, 2001), notes that until 1972, smallpox vaccination was mandatory in the United States. The vaccine killed several children each year and left others brain-damaged. Some children were clearly at risk — those with suppressed immune systems or eczema — but a few had no known risk factors, Dr. Tucker said.

Since Sept. 11 and the anthrax attacks, the federal government has worked to expand its stockpile of vaccine against the possibility that terrorists might get hold of one of the remaining stocks of live virus and mount a biological attack. Until this week, immunization strategies were limited by the amount of vaccine available, not nearly enough for the whole population.

But on Thursday, with the disclosure that the drug company Aventis Pasteur had tens of millions of doses that could potentially be diluted to vaccinate everybody, the terms of the debate shifted sharply. Now the questions are how to decide on a smallpox vaccine plan, and who should be making the decisions.

Some experts and officials say the public should be brought into the debate. Dr. Anthony S. Fauci, the director of the National Institute of Allergy and Infectious Diseases, said in an interview that the current plan — to stockpile vaccine and immunize only those who come in contact with an infected person — should be discussed in detail and compared with other choices.

"It is not an indictment at all of the current plan," Dr. Fauci said. "It's a call for open dialogue." He added that the public "should be given an opportunity to hear an open debate."

But others say rational debate will be difficult.

"Smallpox can devastate a nation, a community, a culture," said James Adams, a terrorism expert who is a senior fellow at the Center for Strategic and International Studies in Washington.

But that prospect would be balanced against the stories of individuals who would inevitably be killed or disfigured by the vaccine.

"It's very hard to overcome that," Mr. Adams said. When people draw back in horror from photographs and stories of vaccine victims, he said, "what one is responding with is visceral fear and not logic."

The 1976 flu vaccine program showed how easily a tide can turn. Soon after the immunizations began, the news media began a national body count of those who had had the vaccine and died. Large studies had shown that the flu vaccine was not particularly dangerous and that the few reactions it caused tended to be mild, like soreness in the arms.

But that did not stop the speculation. Under the headline "The Scene at the Pennsylvania Death Clinic," The New York Post wrote of a 75-year-old woman who "winced at the sting of the hypodermic," then took "a few feeble steps and dropped dead." Her death was almost certainly a coincidence, but the cumulative effect of such accounts was devastating to the immunization program.

Dr. Jeffrey Hunker, a national security expert who is dean of the Heinz School of Public Policy and Management at Carnegie Mellon University, said that with smallpox immunization, the cause-and-effect relationship between vaccination and illness would be far clearer. In rare cases, the vaccine — made from vaccinia, a virus similar to smallpox — would multiply out of control, causing an illness that can be fatal. Victims will first develop oozing, crusting pustules and other signs of smallpox before they die.

Because the chances of a smallpox attack are also highly speculative, Dr. Hunker said, the choice is whether it is worth accepting a few deaths and serious injuries to protect the nation against a threat that may not materialize.

"There is no good established system for making that choice," he said. "Human beings have certain biases in making choices, and unless you're trained, people tend not to make very good choices."

Dr. Arthur Waldron, a professor of history at the University of Pennsylvania who studies war and terrorism, says he worries about making decisions without knowing the likelihood of such an attack.

"The piece of information that's missing for me is what is the dimension of the threat," Dr. Waldron said.

Dr. Tucker, the author of "Scourge," noted that the only publicly available evidence was circumstantial. The only known stocks of virus are held by the United States and Russia, but there are concerns that there may be samples elsewhere, in North Korea and Iraq.

"Even if states had illicit stocks," Dr. Tucker said, "it is hard to understand why they would be motivated to provide them to terrorists."

Dr. Waldron says that when the vaccine was mandatory, the public simply accepted it. And when decisions were made to stop using it, terrorism was not a major concern.

"There were a whole series of dire possibilities that didn't even occur to people," Dr. Waldron said. "I just think that was a real failure of judgment. It's almost a hubristic idea that we can defeat nature and therefore we no longer need to take precautions against smallpox."