An Israeli nurse holds a vial of smallpox vaccine in 2002. People in the United States haven't received a vaccine for smallpox since it was considered eradicated. REUTERS/Havakuk Levison On July 1, workers at the National Institutes of Health notified the Centers for Disease Control and Prevention that they discovered vials containing smallpox in a cold storage room of a Food and Drug Administration lab on the NIH Bethesda campus.

Because it's so infectious and considered a potential biological weapon, there are only two labs in the world authorized to possess variola, or smallpox. One is at the CDC in Atlanta, the other is at the State Research Centre of Virology and Biotechnology in Russia. The NIH is unsure of how the smallpox vials got there and unsure of how long they've been there, but say they appear to date back to the 1950s.

Here's the full statement from the CDC: