Bioterrorism experts fear that other stocks may exist; for example, in 2014 several forgotten vials containing smallpox were found at the National Institutes of Health.

More worrisome, experts say, is the possibility that a terrorist lab or even a sophisticated amateur could use modern gene-editing techniques to rebuild the virus and then unleash it, deliberately or accidentally, on an unprepared world.

Because routine smallpox vaccination stopped after 1980, almost everyone under the age of 40 is unprotected. The disease kills almost a third of people who get it, and is even more lethal to babies.

Finding a medicine was vital because — unlike, for example, measles or whooping cough vaccine — smallpox vaccine is too dangerous to give everyone, said Dr. Peter J. Hotez, former president of the Sabin Vaccine Institute and dean of the National School of Tropical Medicine at Baylor College of Medicine.

The vaccine is now routinely given only to some members of the military, lab workers and others likely to come in contact with the virus in a bioterrorism event. It cannot be given to pregnant women, or to anyone with H.I.V., under cancer treatment or with any other immunosuppressive condition; nor can the vaccine be given to anyone with eczema or several other skin diseases, Dr. Hotez said.

So a medicine like tecovirimat would be useful for treating anyone infected in the first wave of any release of the virus, as well as the millions of Americans who cannot be vaccinated.

Dr. William Schaffner, a professor of preventive medicine at Vanderbilt University Medical School, noted tecovirimat also could be useful for treating monkeypox, which infects humans and has been increasing rapidly in Africa since smallpox vaccination ended.