Expert: Live anthrax may have been sent by military because spores were too tough to kill

Experts believe that there could be a simple explanation behind the Department of Defense inadvertently sending out live anthrax samples.

The spores are incredibly tiny and very tough, and the irradiation procedure used to deactivate them might not have killed each and every one, John Peterson, a microbiology professor who works with anthrax at the University of Texas Medical Branch, told NBC News. Anthrax spores are one micron in diameter, and samples could contain 10 billion spores, making it difficult for an X-ray or gamma ray to kill all the spores. "If a procedure kills 99.999 percent of the bacteria, there would still be 100,000 spores left viable," Peterson said. "They are very difficult to kill."

While anthrax is not fatal if treated in time, spores can be dispersed in the air and stay around for months or years, and it is considered a potential biological weapon. On Wednesday, the Pentagon confirmed samples of live anthrax were sent out to labs in nine states and an Air Force base in South Korea, and at least 26 people who came in contact with the samples are taking antibiotics as a precaution. Investigators say they are looking into the irradiation process that was used to inactivate the spores.

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