The crew was cleaning out a storage room of the Centers for Disease Control and Prevention in Maryland, US, when they found the forgotten box of vials.
The disease was one of the most aggressive viral killers until it was declared officially eradicated in 1980. Since then the disease has supposedly been contained in only two places - one lab in Russia, and the CDC.
After the last official case was recorded in the field in 1977, the World Health Organisation led an effort to collect every lab sample of the virus and consolidate them in Siberia and the CDC facility in Atlanta.
Apparently, these six vials escaped the search.
The smallpox virus is estimated to have killed hundreds of millions of people before its eradication, and was responsible for about one third of all cases of blindness.
The last fatal case of smallpox in the UK was recorded in 1978, after a medical photographer was accidentally exposed to the virus in a laboratory.
Wired reports that the newly found samples might date back as far as the 1950s.
The CDC said in a statement:
"The laboratory was among those transferred from NIH to FDA in 1972, along with the responsibility for regulating biologic products. The FDA has operated laboratories located on the NIH campus since that time. Scientists discovered the vials while preparing for the laboratory’s move to the FDA’s main campus.
The vials appear to date from the 1950s. Upon discovery, the vials were immediately secured in a CDC-registered select agent containment laboratory in Bethesda.
There is no evidence that any of the vials labeled variola has been breached, and onsite biosafety personnel have not identified any infectious exposure risk to lab workers or the public."
The vials are now in the high-containment facility in Atlanta. They will undergo testing and then be destroyed.
"CDC has notified WHO about the discovery, and WHO has been invited to participate in the investigation. If viable smallpox is present, WHO will be invited to witness the destruction of these smallpox materials, as has been the precedent for other cases where smallpox samples have been found outside of the two official repositories.
DSAT, in collaboration with the Federal Bureau of Investigation, is actively investigating the history of how these samples were originally prepared and subsequently stored in the FDA laboratory."
And it's not the first time this has happened. In his 2003 book 'Smallpox', David Koplow found that samples of smallpox had been found three times before the most recent incident. While concerns about bio-terrorism persist, it seems perhaps as likely that if smallpox did return it would be a result of an accident rather than malicious attack.