Washington, July 9 (IANS): Since its eradication in 1980, smallpox officially exists in only two places - the US Center for Disease Control (CDC) in Atlanta, Georgia and its Russian counterpart VECTOR in Novosibirsk.
Now, vials containing the deadly virus have been discovered in a cardboard box in an unsecured refrigerator located on the National Institute of Health (NIH) campus in Bethesda, Maryland.
That refrigerator belongs to the US Food and Drug Administration (FDA) which has conducted some of its research at the Bethesda site since 1972.
FDA researchers discovered the vials labelled "variola" - the name of the virus that causes smallpox - while conducting an inventory of the lab.
"Safety officials have determined that the virus had not leaked and there was no danger to the employees who had found it," CDC said in a statement.
The CDC officials are now attempting to grow the virus in cell culture to determine whether it is still viable, and expect results in two weeks.
The NIH believes that the box that held the smallpox vials dates back to the 1950s but the virus is extremely stable in its powdered form and could still be infectious.
Most experts believe that numerous smallpox virus stocks exist around the world whether in clandestine labs or preserved in tissue, Nature reported.
"This points out the concern that there is the possibility of undisclosed stocks," said virologist Inger Damon, who heads CDC's smallpox research.
According to NIH, it plans to conduct a comprehensive search of all its laboratory spaces as soon as possible.