Government officials say workers cleaning a storage room at a research center made a startling discovery last week decades-old vials of smallpox forgotten in a cardboard box.
The only other known smallpox samples are in super-secure labs in the U.S. and Russia.
Officials say the six vials of freeze-dried virus were sealed and pose no immediate danger. The virus usually has to be kept cold to remain alive and dangerous. The Centers for Disease Control and Preventions says these were stored at room temperature.
Further tests will determine if the samples are dead.
The vials were found at the National Institutes of Health in Maryland and may have been stored there since the 1950s.
One of the most deadly diseases in history, smallpox was declared eradicated in the 1980s.