Although the deadly virus has disappeared from nature, it still exists in laboratories and may be available to rogue nations like Iraq or terrorist groups like al-Qaida. Like using commercial airliners with suicidal pilots as weapons, a smallpox strike — perhaps carried out by a lone, willingly infected "smallpox martyr" coughing his way throughout the country among innocent travelers who then spread the disease unknowingly — is unthinkable until it actually occurs. Unlike Sept. 11, this biological attack would be 10 to 14 days old — the virus' incubation period — before anyone knew it had occurred.
—"Smallpox protection," Bangor Daily News, December 20, 2002

"It's impossible to quantify but I would assess it to be quite low," said Jonathan B. Tucker, an expert in bioterrorism and the author of "Scourge: The Once and Future Threat of Smallpox," (Atlantic Monthly Press, 2001).

But smallpox, which kills about one of three people infected with it, is a particularly worrisome threat because it is easily transmittable. Even those Americans who have been vaccinated are at risk, because the vaccine's protection is believed to last only 15 or 20 years.

And unlike anthrax, which would require that spores be prepared according to precise specifications to infect large numbers of people, a smallpox epidemic could begin with a single infected person — a "smallpox martyr," in the terminology of bioterrorism experts — simply walking through a crowd.
—Sheryl Gay Stolberg, "U.S. Seeks to Stock Smallpox Vaccine For Whole Nation," The New York Times, October 18, 2001