Smallpox is highly contagious, and fatal in 30 percent of cases. Michael Osterholm, a bioterror expert who calls it "the lion king of infectious diseases," said smallpox could be devastating if it got into the wrong hands. "Should it come back today, it would be by far the most serious of all the potential terrorist weapons they could use."

But there is hope. The United States has 4 million to 10 million doses of smallpox vaccine stockpiled, which could go further if diluted. And the Bush administration is negotiating with four drug companies to buy 300 million more doses by next year.

Begins With Flu-Like Symptoms

Smallpox is caused by a virus, variola major. The disease begins — like many other diseases — with symptoms that resemble the flu. Then a rash appears, which at first looks like chicken pox. The rash is contagious and, if not diagnosed immediately, develops into pustules that leave disfiguring scars on those who survive the disease.

Infecting others happens fast — a cough or sneeze can carry millions of virus particles through the air to the next victim. And, because smallpox is a virus, antibiotics — which kill bacteria — are not effective against it. There is no treatment.

Until 1972, doctors in the United States routinely vaccinated children when they were one year old. Most Americans over the age of 30 have a scar on their arm or leg from the childhood shot, but the vaccination eventually wears off. Americans born after 1972 have never been vaccinated at all.

"We have a very susceptible population, which means that once it were to get into that population, it would not only take hold, but its ability to be transmitted on to additional generations with large increases in numbers of each of those generations is a very real possibility," said Osterholm, who is director of the University of Minnesota's Center for Infectious Disease Research and Policy.

Who Has It?

After smallpox was eradicated, two laboratories in the world were allowed to keep samples of the virus for research purposes — the Centers for Disease Control in the United States and a laboratory in the former Soviet Union.

But Osterholm and other biological weapons experts believe that other countries have obtained the virus.

"Any belief that smallpox virus is only in two secure laboratories in the United States and the former Soviet Union is just a myth," said Osterholm. Judging from audits and reports from sources and defectors, Osterholm believes it is "certain" that North Korea's hardline Communist government has the virus, and he feels there is "little doubt" that Iraq has it, too. He is also concerned about the prospect of former Soviet biological weapons engineers hiring themselves out to the highest bidder.

Smallpox Virus Is Easy to Disseminate

Smallpox is one of the smallest living organisms known, making it very easy to aerosolize. Terrorists could release the virus into the air at a crowded place like a railway terminal, an airplane, or a stadium. Or a suicide terrorist could infect himself, then walk through the streets coughing and sneezing, infecting passers-by.

The good news is that the vaccine is effective even if administered after exposure. "We have data showing that if you receive the vaccine up to several days after your exposure, much like with the rabies vaccine in dog bites, you can still be protected," said Osterholm.

Tests have shown that the government's 4 million to 10 million doses of vaccine, which are stored at a secret location, are still effective even though they are 30 years old. The National Institutes of Health are conducting tests to see if the vaccine can be diluted fivefold to increase the number of people it could protect, and early results have been favorable.

The government plans to purchase enough doses to treat every American, and Ridge suggested officials would consider resuming routine vaccination for children.

The vaccine has rare, but serious, side effects that cause one death for every 1 million people treated.

ABCNEWS' Dr. Nancy Snyderman contributed to this report.