In the summer of 1977, an influenza epidemic began to spread through China; by fall, it was in Russia, and over the winter, it spread across the Northern Hemisphere. That was very normal, of course. Winter is flu season, and this was not a bad flu, as flu seasons go: It affected mostly people younger than 25.

Everything seemed typical, until virologists got a chance to analyze the strain. Once they did, they were disturbed—and also, instantly clear on why the virus was affecting only young people. Because what they were looking at was not the virus they would have expected to see, a slightly mutated variant of the flu that had circled the world in the previous 12 months. Instead, it was almost identical to a flu strain that had not been seen in the wild in more than 20 years. That was why people older than 25 were not made sick by it; they had developed immunity to it the last time it came around. Equally, that was why people younger than 25 were ill: They had never had the chance to experience this flu as either a natural infection or a component of a vaccine.

In the annals of epidemics, the “Russian flu” is only a curiosity, and not a disaster, because that mysterious strain caused only mild illness; if it had been virulent, the lack of immunity in the world could have led to a terrifying pandemic. But it is also a cautionary tale, because world health authorities eventually concluded that the 1977 eruption was not a natural event. The only way a flu strain so like an extinct one could have reappeared, they said, was for it to be the extinct one—preserved in some lab’s freezer, and released in some accident that has never been acknowledged.

This story is important, and not just because that flu strain eventually did cause a pandemic: It was one of the viral parents of the mild but very infectious flu that swept the globe in 2009. It’s important because no matter how hard campaigners work to eradicate diseases in the outside world, the packrattery of science, which holds on to everything because someday it might be useful, remains their Achilles heel.

View Images Rinderpest virus. Photograph courtesy Wikimedia Commons, CC.

For that reason, a warning just issued by the World Organization for Animal Health (and published ahead of print in the monthly journal of the Centers for Disease Control and Prevention) is worth listening to. Superficially, the warning is a caution to scientists to clear their freezers of rinderpest, an economically devastating cattle disease that was eradicated in 2011.

But at a deeper level, it is a reminder that no matter how hard thousands of people work to eliminate a disease—as in the almost 30 years and billions of dollars that have gone toward the still-elusive eradication of polio—it can all be undone by one misstep. And that misstep has the potential to echo unpredictably down decades.

In the just-published piece, the authors explain how a robust eradication campaign can’t help but make itself vulnerable:

Throughout the eradication campaign, in affected and nonaffected countries, rinderpest material became widely disseminated in diagnostic laboratories, vaccine production facilities, and research institutes. While efforts were focused on eradication, less thought was probably given to what would happen to this material after eradication. In 2015, although natural infections in animals have been eradicated, live rinderpest virus, vaccines, and genetic material remain stored in scientific institutes across the world.

As part of the campaign to eradicate rinderpest, the 180 members of the world organization (which is usually referred to by its acronym in French, OIE) agreed to destroy all stocks of material that contains the rinderpest virus. But, the authors report, two surveys taken in 2014 and 2015 reveal that 24 countries are still holding on to “rinderpest materials,” and 23 of them possess live virus. (And in a sign of how easy it is to miss this stuff, one country said in 2014 that it did not possess the virus, and then in 2015 said, Oh wait, it did.)

This, the authors say, is “an unnecessarily high risk.”

Rinderpest was the second disease eradicated from the world, after smallpox. The third, and the one so many hopes are pinned on, should be polio. The international eradication campaign has been focused on eradicating the disease from the wild, but from the start, a small portion of what it calls its “endgame planning” has focused on what might remain in labs after the disease has been forced out of circulation. That is a more complicated task than just looking in freezers for things labeled “polio”; poliovirus lives in the gut, so there is a small possibility that any fecal sample stored anywhere in the world might contain it. Dr. Neal Nathanson, who as a young CDC disease detective participated in the first polio-vaccination campaign in 1955, wrote in Science in 2002Science in 2002: