NANTES, France—A hospital nurse soaked a bandage in a colorless liquid containing viruses drawn from a toxic sewer in Paris, a well in Mali and a filthy river in India. Then she daubed it gently on an elderly woman’s severely burned back.

“It’s healing,” said Ronan Le Floch, the doctor overseeing the burned woman’s care. The painful wound’s greenish tinge, the telltale sign of a potentially deadly bacterial infection, had vanished.

The liquid treatment was a cocktail of about one billion viruses called bacteriophages, which are the natural-born killers of bacteria. Little known among doctors in the West, phages have been part of the antibacteria arsenal in countries of the former Soviet Union for decades.

Doctors in the U.S. and much of Europe stopped using phages to fight bacteria when penicillin and other antibiotics were introduced in the 1940s. Now, though, Western scientists are turning back to this Stalin-era cure to help curb the dramatic growth of bacterial resistance to antibiotics.

That resistance has become one of the biggest health-care crises of our time, and public-health experts are desperate for new bacteria-fighting weapons, even old ones like phages. “We are at risk of losing the race here,” says Tom Frieden, director of the U.S. Centers for Disease Control and Prevention. “This has the potential to undermine much of modern medicine.”