But Hatfull has spent decades studying phages that attack mycobacteria, the group to which the girl’s life-threatening microbes belonged. Her doctors wanted to know whether he had anything in his arsenal that might kill those particular strains. He looked in his database—and found Muddy.

In laboratory tests, Muddy efficiently destroyed the exact strain of M. abscessus that was itself destroying the London patient’s body. “It was good that we found one,” Hatfull says. “But it was bad that we only found one,” because bacteria can easily evolve to resist any single phage.

His team eventually found two more phages— BPs and ZoeJ—that had the potential to kill M. abscessus, but weren’t doing it very well. Some phages kill the bacteria they infect by reproducing frantically and bursting out in fatal fashion, but others opt for a more tranquil existence of harmlessly hiding in their hosts. BPs and ZoeJ naturally go for the latter path, so Hatfull’s team modified them by deleting the gene that keeps them peaceful. Unrestrained, these modified microbes could kill M. abscessus as well as Muddy.

Read: The viruses that eavesdrop on their hosts

Last June, the London team started injecting all three phages—one natural and two modified—into the patient. She didn’t experience any major side effects, and after a month of twice-daily doses, the infection in her chest began to disappear. Shortly after, her liver cleared up. After six months, almost all the other lesions had faded. “It’s not like she’s out of the woods, in the sense that she has cystic fibrosis and a new set of lungs,” Hatfull says, “but she’s in very good general health.”

As with any single case of medical success, it is impossible to truly know whether the supposed treatment was what eventually saved the patient: That’s why doctors run clinical trials. But Benjamin Chan of Yale University says that this “fantastic” study “very nicely shows a probable impact of the phages.” After all, the patient’s infections clearly weren’t going away on their own, and they weren’t responding to other treatments.

Phages were commonly used to treat infections in the 1920s, and though they’re still used in Russia and parts of eastern Europe, they largely fell out of favor in the West. But they’ve stepped back into the limelight after a growing line of dramatic success stories. The most famous case involves Steffanie Strathdee, an epidemiologist who led the hunt for phages that ultimately cured her husband, Tom Patterson, of a life-threatening infection. Such successes have prompted a renewed interest in phage therapy, especially in the era of antibiotic-resistant superbugs.

The London patient’s case is a milestone—she is the first person to be treated with phages that have been genetically engineered. “It requires trust to take a leap off the edge into completely unknown medicine,” says Hatfull, who appreciates that many people might be unnerved by his team’s work. “The idea of using a virus in the first place is challenging, let alone messing around with it,” he acknowledges.