By Yasemin Saplakoglu -  Live Science

"There is no indication that anyone has been exposed."

While cleaning out a freezer in a Pennsylvania vaccine research facility, a lab worker discovered several frozen vials labeled "smallpox," launching an FBI and CDC investigation, according to recent news reports.

Only two labs in the world — a CDC lab in Atlanta, Georgia, and a Cold War-era bioweapons lab in Russia — can legally hold samples of the deadly virus, Live Science previously reported. In the recent incident, a total of 15 vials — five labeled "smallpox" and 10 labeled "vaccinia" (the virus used in smallpox vaccines) — were discovered in the Merck facility, according to an "official use only" alert sent out to the Department of Homeland Security leadership on Tuesday (Nov. 16), and obtained by Yahoo News. 

"There is no indication that anyone has been exposed to the small number of frozen vials," the CDC said in a statement to Yahoo News. "The laboratory worker who discovered the vials was wearing gloves and a face mask." The CDC, its administration partners and law enforcement are now investigating the matter, according to the statement. "We will provide further details as they are available," the CDC said.

Smallpox is a highly contagious disease caused by the variola virus; it "was one of the most devastating diseases known to humanity," causing millions of deaths before the world's most successful vaccination campaign led to its eradication in 1980, according to the World Health Organization. 

Smallpox is a highly contagious disease caused by the variola virus; it "was one of the most devastating diseases known to humanity," causing millions of deaths before the world's most successful vaccination campaign led to its eradication in 1980, according to the World Health Organization.