BLOOMINGTON -- In the jungle of infectious diseases, smallpox is the 800-pound gorilla.

It is unlikely to attack. But if it does, watch out.

Smallpox was eradicated as a disease worldwide in 1980, and vaccinations of the general public stopped. But samples were kept for research, and federal officials are increasingly concerned terrorists may use them as a weapon.

Half of all Americans have been born since routine immunization stopped in 1972. Everyone else has little remaining protection, so federal officials have proposed calling for mass inoculations if the disease returns.

Dr. John Lumpkin, director of the Illinois Department of Public Health, said it may be a month before the state's plan is final and sent to county health departments for their response.

Bob Keller, director of the McLean County Health Department, wonders whether mass vaccinations would be an overreaction -- in part because of the risks of side effects.

Brian Wilkinson, a professor of microbiology at Illinois State University, has the same concerns.

Both are bothered by an apparent change in philosophy in how public health officials would respond to an outbreak.

Until now, if someone locally was diagnosed with an infectious disease, he or she would be inoculated, then people in close contact and then those whom that group may have exposed.

Under the federal proposal, that so-called ring approach would be replaced by mass inoculations offered to everyone in and near the Twin Cities.

It would be a logistical nightmare for local health officials. Among the questions, say Keller and Wilkinson: When would the vaccine arrive? Where would inoculations be given? In what order would people be inoculated? Who would be trained to give thousands of inoculations? Would law enforcement be ready in case of a panic?

Training has begun in Chicago and Springfield, and soon those people will train others because -- unlike traditional needles -- the smallpox vaccine needle is two-pronged, and is used to poke the skin 15 times within a few seconds.

Initially, it likely would be given to "first responders" -- paramedics, police officers, firefighters, and perhaps emergency room doctors and nurses.

Lumpkin views the planning as "minor modifications" to the bioterrorism-response planning state and local health and public-safety workers have been doing since fall 2001.

But Keller and Wilkinson see a bigger challenge.

Either way, strategies used for years to control natural outbreaks may not apply to an engineered outbreak. Since the terrorist attacks of Sept. 11, 2001, the U.S. government has increased its smallpox vaccine reserves to a half-billion doses.

"First, I think the mass inoculation approach is a recognition that we have enough vaccine on hand to inoculate the entire population," said Lumpkin, adding that smallpox spreads quickly and kills perhaps one-third of its victims.

Also, people are more mobile than in the past, he said.

"We may not know everyone who was exposed," Lumpkin said. "So the way to stop it cold would be mass vaccination."

Keller and Wilkinson raise another concern: The smallpox vaccine is old and has a higher risk of side effects than other vaccines, such as those for chicken pox and hepatitis B. If 1 million people were vaccinated, 530 would develop non-life-threatening complications, 15 would develop life-threatening complications and one or two would die, Wilkinson said.

Some of those people would be the "first responders" expected to help during a bioterrorist attack, Keller said.

Lumpkin concedes the vaccine "is not built to 21st century standards" and would pose a great risk to some people, including those with HIV and patients receiving chemotherapy. A new vaccine may have a lower rate of side effects, but it is several years away.

Unlike anthrax, smallpox would be difficult to use as a weapon, Wilkinson said. But preparation -- not overreaction -- is good because no one knows what will happen.

"The risk of attack is small," he said. "But if there is a release, it could be catastrophic."