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Signs like this were used to show which houses had received the smallpox vaccine

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Surprise! While preparing to move to a new lab, scientists at the U.S. National Institutes of Health discovered something unexpected in the old one: little vials containing smallpox virus. By international agreement, samples of smallpox are only supposed to be kept in one of two labs in the world. One is part of the U.S. Centers for Disease Control and Prevention in Atlanta. The other is the VECTOR Institute in Russia.

There's no evidence that anyone has taken or used the vials, according to a statement from the Centers for Disease Control and Prevention. In addition, biosafety staff at the NIH don't think there's any risk that anybody got sick from the vials. The CDC believes the vials may date from 1950s, when many labs held smallpox for research. But the centers' Division of Select Agents and Toxins is still investigating who prepared the samples and how they ended up in this unused NIH freezer.

The surprise vials were labeled "variola," the scientific name for smallpox, ABC News reports. A number of other vials in the same freezer weren't labeled at all. When NIH scientists first discovered the vials on July 1, they secured them and alerted the CDC, which sent three experts to come pick the virus up in a government plane. Tests confirmed the six labeled vials contained smallpox DNA, while the other vials didn't.

The CDC plans to perform further tests to see whether the virus in the vials is "alive" enough to grow in petri dishes in lab. After that, it plans to destroy the viruses. 

Vaccination campaigns eradicated smallpox from the world in 1979. That same year, labs around the world agreed to either destroy their own variola samples or to send them to one of the two approved labs. As far as officials could tell at the time, all stockpiles outside of the CDC and the VECTOR Institute were gone by the early 1980s.

Most people now don't even get vaccinated against the disease. Only researchers who work on smallpox, certain healthcare workers, and other people who are at high risk get the vaccine, because it has some rare but potentially deadly side effects; balance that against the fact that smallpox has been eradicated in the wild, and no organization recommends routine vacciation.  Governments do often hold stockpiles of the vaccine, which doesn't contain variola itself, so it doesn't break the international agreement. There's no cure for smallpox, which can kill from 30 percent to nearly all those who contract it.

[ABC News, Centers for Disease Control and Prevention]