A government employee made an unsettling find on July 1 while cleaning out a laboratory refrigerator: it contained six vials of smallpox virus stashed in a cardboard box, likely forgotten since the 1950s, as Nature News reports. (Scientific American is part of Nature Publishing Group). The discovery concerned health officials because only two laboratories (in the U.S. and Russia) are authorized by the World Health Organization to keep smallpox virus stocks, which they are to use only for research.

The recovered vials have aroused speculation and fears about the potential for other forgotten smallpox samples elsewhere in the world, hidden in laboratory freezers or possibly even preserved in human corpses or tissue. The mere thought of a renewed smallpox outbreak makes anyone shudder, yet most people today live untouched by the ghastly infectious disease, one of the deadliest in history. A global immunization campaign officially eradicated the virus in 1980, although the last case in the U.S. occurred much earlier, in 1947. The last documented human infection occurred in England in 1978.

Before widespread vaccination, smallpox menaced humans for centuries. The virus (Variola) survives only in humans and spreads via person-to-person contact and saliva droplets. It causes high fever and fatigue as well as infamous fluid- and pus-filled blisters that cover the body—especially the face—and often leave deep scarring in survivors. Smallpox has two variations: Variola major, which killed about 30 percent of its victims, and Variola minor, which had a 1 percent mortality rate. Experts estimate that at least 300 million people died from smallpox in the 20th century.

Thankfully, the smallpox vaccine has virtually wiped out the virus. It’s also possible that recently developed antiretroviral drugs could protect against smallpox, although the compounds have yet to be tested on an infected human. Given the small risk of a future outbreak, the World Health Organization has even debated destroying the last two known stocks of smallpox in the U.S. and Russia. It would deliver a final blow to the virus…although, as this week’s discovery revealed, perhaps a bit more of the virus endures than we think, secured or unsecured, stashed away somewhere in a cardboard box.

Related stories on smallpox:
Vials of Smallpox Virus Found Unsecured in Government Lab
Uncertain Threat: Does Smallpox Really Spread That Easily?
World Health Organization to Decide the Fate of Smallpox Stocks
The Eradication of Smallpox