Why Smallpox Is so Dangerous As Vials Found in Pennsylvania

Small glass containers bearing labels that read "smallpox" have been found in Pennsylvania, sparking an investigation by health officials.

It is understood that the FBI and the Centers for Disease Control and Prevention are looking into the discovery at a facility owned by pharmaceutical company Merck. Fifteen vials were found at the site outside Philadelphia, according to Yahoo! News.

In a statement to CNN, the CDC confirmed that it was investigating the vials and said they had been found in a freezer by a worker who was cleaning the appliance. The CDC said there was no indication that anyone had been exposed to the vials' contents.

Smallpox, a deadly contagious disease caused by the variola virus, killed millions of people around the world before it was eradicated some decades ago.

The most common and severe form of smallpox, variola major, had a fatality rate of around 30 percent, according to the U.S. Food and Drug Administration.

Symptoms begin around 12 to 14 days after exposure, and usually start as a high fever and severe headache and backache, the FDA states. Patients then erupt in a blistery rash that scabs over after a couple of weeks.

Smallpox can be spread from person to person through sneezing, coughing, breathing, contact with scabs or fluid from the rash blisters, and even from touching a patient's personal items, according to University of Michigan Health.

There is no proven effective treatment for smallpox once someone contracts it. Vaccines can be used to prevent smallpox. The first vaccine was developed in 1796 by the British physician Edward Jenner.

Thanks to a concerted international vaccination and surveillance effort in the 1960s, the disease was declared eradicated in 1980 by the World Health Organization. As a result, vaccination against smallpox is no longer recommended for the general public.

The WHO considers the eradication of smallpox "among the most notable and profound public health successes in history." The last known case occurred in Somalia in 1977.

It is understood that the variola virus is still kept in some medical laboratories under strict biosecurity measures.

According to the CDC, there are only two WHO-designated medical laboratories in the world where the smallpox virus is stored: a CDC site in Atlanta, Georgia; and the State Centre for Research on Virology and Biotechnology in Koltsovo, Russia.

However, there are concerns that the smallpox virus could be being kept elsewhere, leading to the possibility of an accidental or purposeful release. The CDC says "the effects of such an event could be devastating."

A stock photo shows a person in a biohazard suit working in a lab. Smallpox samples are stored in two highly secure research labs—one in Russia and the other in the U.S. luchschen/Getty