Smallpox is an ancient scourge, but it is the more modern curse of international terrorism that has brought this infectious disease back into the nightmares of those charged with protecting public health and safety. In the post 9/11 era, prudent policy dictates that such concerns be taken seriously.

In announcing a program for mass vaccination of military, public safety and medical personnel, President Bush has charted a middle course between complacency and vigilance. His easiest option, but perhaps the most dangerous, would have been to hold to the status quo, the belief that death and disfigurement from smallpox belong firmly in the past.

The disturbing truth is: Nobody really knows for sure. The last case of smallpox in the United States was reported in 1949; the last naturally occurring case in the world was in Somalia in 1977. The virus was said to be eliminated in 1980, except for stocks kept in a few laboratories.

But perhaps one of those laboratories is in Iraq, or is operated by some shadowy terrorist group. As the U.S. Department of Health and Human Services says on its Web site, "We cannot quantify the threat that smallpox would be used as a bioweapon, but we do know that the consequences of its use could be great." As for Iraq, it says: "It is possible, but not confirmed, that Iraq possesses the virus that causes smallpox."

That assessment may be as good as it gets -- and undoubtedly it won't be good enough for some who will see the vaccination directive as a costly (and, for a few individuals, dangerous) overreaction. But the administration has, in fact, come up with a balanced policy in dealing with the potential threat. Although enough vaccine has been stockpiled to vaccinate every American, it is not recommended that everyone receive the shot.

In this wise way, potential side-effects in the general population are kept to a minimum and the nature of the threat is kept in perspective. Although ordinary citizens can be vaccinated if they insist on it (although not immediately), the federal government has rightly made America's front-line personnel the priority -- members of the military, hospital workers and police, firefighters and paramedics. The Allegheny County Health Department will do its part, expecting to vaccinate as many as 10,000 people. They will be part of some 11 million Americans who are expected to be vaccinated by next year.

Statistically some adverse reactions are sure to occur, but in the scheme of things they should be few and the benefit accruing to the security to public health will be great. Judging that trade-off is the eternal dilemma facing authorities whose duty is to protect a mass population.

As it is, exaggerated fears about the process are contradicted by the fading signs of smallpox vaccinations on the arms of older Americans who once routinely received them. As an act of reassurance, President Bush has volunteered to be one of those vaccinated.

Never before has such a program been undertaken in the name of "homeland security." But if the administration has erred at all, it is on the side of caution -- and that is no bad thing. At the very least, the evil ambitions of America's enemies have been limited.