On Friday, the Centers for Disease Control and Prevention (CDC) confirmed that two of the six unaccounted for vials of smallpox found in an unused storage room in Maryland contained living samples of the virus. The vials were found to have “evidence of growth,” and scientists are currently working “through the night” to determine the status of the remaining four vials. The NIH is now searching the rest of its building in order to confirm that no other vials filled with deadly viruses remain.

Previously, it was believed that the last remaining vials of smallpox existed in only two laboratories: one in Georgia, and one in Siberia, Russia. The discovery of these six unaccounted for vials has come as a shock, but the CDC assures that there is no evidence that any of the vials have been breached, CNN reported. The vials were found in an unused storage room by National Institutes of Health employees as they were preparing to move the lab from Maryland to a different location. “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,” the CDC's press release said.

If more viable smallpox are found, the World Health Organization “will be invited to witness the destruction of these smallpox materials, as has been the precedent for other cases where smallpox samples were found outside of the two official repositories,” the press release said. CDC Director Tom Frieden called the incident a “problem in inventory control,” and ensured that testing would continue for the remaining four vials.

Unfortunately, misplacing vials containing living specimens of a deadly virus is not an isolated event. In April, the Pasteur Institute in Paris confirmed that they had misplaced more than 2,000 vials containing fragmented samples of the SARS virus. As of yet, there is no news on the location of these vials, but officials ensure that the vials pose no potential to infect because they only contain parts of the virus, International Business Times reported.

The last known U.S. outbreak of smallpox occurred in New York in 1947. Since 1980 the disease has been considered eradicated, but for centuries before, smallpox was responsible for the deaths of countless individuals, killing between 300 and 500 million in the 20th century alone. Smallpox causes an extensive rash and high fever. Smallpox variola, the strain found in the vials, is a less severe form of the virus. Historically, it has a death rate of about one percent, which is extremely low compared to Smallpox major, which has a death rate of 30 percent. For those that survive a bout of smallpox, extensive scarring of the skin is almost guaranteed. Eye problems and arthritis are also common complications found in survivors of the virus.