But this error is not the first involving the U.S. government and shipments of deadly germs to unsuspecting scientists. In 2014, the CDC closed two labs and imposed a temporary moratorium on shipping deadly pathogens after a string of incidents.
Government facilities have mistakenly sent, mislabeled or lost anthrax, smallpox, botulism, brucella and dangerously contaminated
CDC officials have said they won't know what actions to suggest to prevent future mistakes until they finish their investigation into the shipping errors discovered last month.
Though these events rarely cause public health risks, the mishandling of dangerous biological agents has caused concern at least half a dozen times in the last decade. In 2014, the CDC issued a report on other incidents involving hazardous bacteria and viruses dating to 2006. Some highlights:
In 2006, a government biodefense lab failed to ensure that vials of anthrax were rendered inert — essentially lifeless and incapable of growth — before sending the samples to another facility.
The CDC also caught its bioterrorism labs storing anthrax in unlocked refrigerators and transporting live samples in Ziploc bags. CDC officials appeared before a congressional oversight panel in 2014 and said the lapses in security were "completely unacceptable" and "should never have happened." More than 80 CDC employees were potentially exposed to anthrax because of those lapses, but none was infected.
The government uses anthrax in research to develop tools to detect potential biological terrorism.
Smallpox, one of history's deadliest diseases, was eradicated in 1977 after a worldwide vaccination movement. The last U.S. case appeared in 1949, and Somalia saw the world's last naturally occurring case, according to the CDC.
To ensure the virus never reappeared to infect people, laboratories around the world were ordered to destroy stored samples of the virus — dead or alive.
Only two facilities, the CDC in Atlanta and a lab near Novosibirsk, Russia, could keep smallpox-causing viruses in case the disease resurfaced.
That's why it was a major shock when six small vials labeled "variola" turned up in an unused storage room at a research facility in Bethesda, Md., in 2014. Variola is another name for the deadly and long-eradicated virus that causes smallpox.
The vials dated to the 1950s. Apparently, someone had freeze-dried the virus, packed it away in a cardboard box and forgot about the vials hidden in the storage room.
Officials speculated that the vials may have once belonged to a National Institutes of Health lab that left the box when the facility changed hands to the Food and Drug Administration.
The last known cases of smallpox resulted from a 1978 lab accident in England.
This year's avian flu outbreak has affected more than 43 million birds in the U.S., according to the Department of Agriculture. And though it is rare and hasn't happened in the current outbreak, people can become infected by fowl carrying the disease.
About 60% of the people who contract the strain of the disease known as H5N1 die, according to the CDC.
In 2014, a CDC lab accidentally mailed vials of a benign bird flu contaminated with H5N1 to the Department of Agriculture. The department didn't realize the mistake until several chickens died. The flu was contained in the lab and none of the workers was infected.
In 2006, a CDC lab mistakenly shipped live botulism bacteria, which create a potentially deadly nerve toxin, to another facility, which the CDC did not identify in its report.
The incident was one of five accidents involving dangerous pathogens recounted in the 2014 report by the CDC outlining mistakes and procedural errors at government labs.
Another incident mentioned in the CDC's 2014 investigation into mishandled pathogens involved brucella bacteria in 2006. A CDC lab mistakenly sent what were thought to be vaccines to another facility that was not identified in the 2014 report. The mislabeled samples actually contained live brucella.
The germ can cause an infection called brucellosis, which is the most common work-related infection contracted by scientists. People who get sick from brucellosis can suffer from flu-like symptoms, including fever and muscle pain, but some effects can last for years and cause arthritis or depression.
Over the years, the CDC has closed labs, issued temporary halts to shipping deadly pathogens and completed numerous investigations because of incidents detailed in its 2014 report and the live anthrax shipments discovered in May.
It is still unclear why those samples were still alive when researchers thought the spores were dead.
The CDC and Pentagon "are continuing to investigate the circumstances around the shipment of the samples," said Jason McDonald, a spokesman for the CDC. "It's premature at this point to speculate on what, if any, additional procedures may need to be put in place in the future."