Swine flu, or H1N1, had been dead for 20 years when it suddenly re-emerged in 1977 with a curious twist. The new strain was genetically similar to one from the 1950s, almost as though it had been sitting frozen in a lab since then. Indeed, it eventually became clear that the late-70s flu outbreak was likely the result of a lowly lab worker’s snafu.

Lab accidents like that are extremely rare. Still, two scientists are now arguing that it’s not worth continuing to create new, transmissible versions of deadly viruses in labs because the risk that the diseases will escape and infect the public is too great.

The H5N1 avian flu killed two dozen people in Hong Kong in 1997. It has only killed about 400 people worldwide since then, though, because it doesn’t pass easily from human to human.

In recent years, scientists have found a way to make H5N1 jump between ferrets, the best animal model for flu viruses in humans.