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By Maggie Fox

U.S. military and health officials are trying to figure out how anthrax from a batch containing live spores got sent to labs in nine states, Australia and South Korea. Here’s what you need to know about the blunder:

Is anyone going to get infected?

It’s unlikely, the Centers for Disease Control and Prevention says. The samples were irradiated to kill them and even if a few spores did survive, the samples would have been packaged for shipment. It takes several thousand spores to infect a person and they have to be inhaled, eaten or get onto the skin to do that. Anyone who’s handled the samples out of the packaging has been offered antibiotics but the CDC and other experts say it’s really an abundance of caution. Antibiotics can prevent an anthrax infection from ever taking hold.

How did this happen?

The U.S. military and experts outside the military say it appears that some of the spores lived through the radiation process used to deactivate them. John Peterson, a microbiology professor who works with anthrax at the University of Texas Medical Branch, says the X-rays or gamma rays used to kill anthrax spores might not get every little one.

It’s hard working with germs that are invisible to the human eye, experts note. Even in hospitals, where people expect to be exposed to infectious agents, mistakes happen multiple times every day. For instance, more than 450,000 Americans are infected with potentially deadly Clostridium difficile every year. “What’s required is eternal vigilance,” says Ken Berns, professor emeritus of biology and genetics at the University of Florida. “People have to constantly be aware of what they're doing, thinking about each step that they do, and there have to be checks on all of the procedures as they’re carried out.”

Does this happen often?

Government sources tell NBC that more than 3,600 transfers of so-called select agents — those that might be used in a biological weapon — have been made without anyone getting infected. NBC News learned there were 300 shipments last year alone, mostly via FedEx.

But there have been lab mishaps. Last June, the CDC said live anthrax was accidentally sent internally from one lab to the other without the proper precautions having been taken.

Live smallpox was found in a freezer at the National Institutes of Health last year. Smallpox is only supposed to exist in two places in the world, locked up tight in safes, because the highly deadly virus has been eradicated since 1979. No one got sick in either incident.

Why were they shipping anthrax in the first place?

Anthrax is considered one of the top bioterrorism threats. It’s dangerous because it forms tiny, hard spores that can float in the air, settle on surfaces, and persist for years until someone touches them or breathes them in and they get activated by the moist human tissue. In 2001, someone sent anthrax spores through the mail to NBC News, other media outlets and to Congress, killing five people and making 17 sick. They included two postal workers and two people who may have handled contaminated mail.

For this reason, government and private labs want to be able to test for anthrax, including using “sniffers” that can find it in places like train stations and shopping malls. They need samples of real spores and bacteria to validate their tests.

Critics such as Richard Ebright of Rutgers University say there are too many labs doing this, however. “There are approximately 1,500 US laboratories authorized to work with fully active, fully virulent, biological weapons agents,” he told NBC News. “This number is too large by a factor of 10 to 20.”

Is it dangerous to ship biological agents by FedEx?

FedEx ships such agents under the supervision of the U.S. government. “There are very stringent regulations about how you have to package agents,” Berns said. “Now, the ones that were being sent out — yesterday's incident — should have been less of a problem because they were supposed to be inactive.” But Peterson and other experts say even inactivated spores would have been packed carefully.

People may remember the 2001 anthrax attack was carried out by mail and the infected postal workers were unknowingly breathing in spores from envelopes that passed through a mail distribution center in Washington, D.C. But Gigi Kwik Gronvall, a senior associate at the UPMC Center for Biosecurity in Baltimore, points out that those envelopes were meant to leak.

“They were sent with intent to harm in 2001,” she points out “While we have heard that the samples were not improperly inactivated in this case, they were probably properly transported, meaning that they were still sent in containment and there was never any danger to anyone carrying the boxes or anywhere along the chain of delivering the packages,” she said.

“The problem in 2001 was the anthrax was put without any packaging into envelopes.”