What you need to know about the anthrax blunder

A general view of the Centers for Disease Control and Prevention (CDC) headquarters in Atlanta, Ga. on Sept. 30, 2014. (Photo by Tami Chappell/Reuters)
A general view of the Centers for Disease Control and Prevention (CDC) headquarters in Atlanta, Ga. on Sept. 30, 2014.

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.

RELATED: Live anthrax mistakenly sent to US labs

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.

Read more on NBCNews.com.