Authorities are investigating why a Pennsylvania laboratory stored deadly disease samples.

The FBI and U.S. Centers for Disease Control and Prevention (CDC) want to know why Merck’s kept five vials marked as “smallpox” at its vaccine research facility in Montgomery County.

A lab worker reported the surprise discovery after cleaning out a freezer and finding 15 vials. Since he protected himself with gloves and a mask he is not considered to be at risk of infection, according to the New York Post.

“CDC, its administration partners, and law enforcement are investigating the matter and the vials’s contents appear [to be] intact,” a CDC representative said according to the paper.

Smallpox is an infection caused by the variola virus, and symptoms include a fever and progressive skin rash.

Smallpox was officially eradicated worldwide back in 1980. The last case was reported in 1977, and only two research laboratories still keep small samples according to the U.S. National Library of Medicine’s online information service.

Smallpox’s origin is unknown. The first written description of a similar virus appeared in China during the 4th Century. The virus killed an estimated 300 million people during the 20th Century.

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