The Department of Defense says an attempt to ship inactive anthrax samples resulted in live samples being sent to labs in nine U.S. states and to a U.S. Air Force base in South Korea.

Fears of exposure to the potentially deadly disease prompted officials to advise four civilian workers to get preventive care; more than 20 military personnel are also being monitored. The samples were sent via commercial shipping companies, but the Pentagon says there is "no known risk to the general public."

The spores were supposed to have been killed by being irradiated. But at least one lab, in Maryland, reported receiving spores that were still alive. The Centers for Disease Control and Prevention is investigating the accidental shipment.

From NPR's Science Desk, Geoff Brumfiel reports:

"The anthrax came from the Army's Dugway Proving Ground in Utah, which conducts biosecurity research. The samples were apparently sent to labs in nine states as part of ongoing efforts to develop field tests for anthrax.

"Tests are usually conducted with dead samples of anthrax, and an investigation is now under way to learn why live samples were sent instead. Anthrax are highly infectious bacteria that can travel through the air and infect people.

"So far, the Pentagon says it appears nobody has become sick from the mistake. It has suspended all shipments of biological agents from its labs until an investigation is completed."

The military says 22 personnel were potentially exposed to the live sample at a U.S. base in South Korea.

From Seoul, NPR's Elise Hu reports:

"The military destroyed the anthrax on Wednesday after the sample, which was thought to be inactive, was suspected of being live. None of the personnel in contact showed signs of exposure. But they were given exams, antibiotics, and in some cases, vaccinations.

"The military stresses there was no risk to the public. The exposure happened in a contained lab environment on the Osan Air Base, which is 65 miles south of Seoul. It's one of a number of bases that houses the estimated 28,000 American military personnel based in South Korea."

According to the CDC's fact page on anthrax, it's caused by "rod-shaped bacteria known as Bacillus anthracis," is found naturally in soil, and affects domestic and wild animals around the world.

Anthrax can cause severe illness, the CDC says — the agency adds that while the disease can spread through spores, "anthrax is not contagious, which means you can't catch it like the cold or flu."

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