For decades, public health experts have warned that the overuse of antibiotics was reducing the effectiveness of drugs that have lengthened life spans by curing bacterial infections once commonly fatal. But lately, there has been an explosion of resistant fungi as well, adding a new and frightening dimension to a phenomenon that is undermining a pillar of modern medicine.

This is similar to concerns that resistant bacteria are growing because of excessive use of antibiotics in livestock for health and growth promotion. As with antibiotics in farm animals, azoles are used widely on crops.

“On everything — potatoes, beans, wheat, anything you can think of, tomatoes, onions,” said Dr. Rhodes, the infectious disease specialist who worked on the London outbreak. “We are driving this with the use of antifungicides on crops.”

Dr. Chiller theorizes that C. auris may have benefited from the heavy use of fungicides. His idea is that C. auris actually has existed for thousands of years, hidden in the world’s crevices, a not particularly aggressive bug. But as azoles began destroying more prevalent fungi, an opportunity arrived for C. auris to enter the breach, a germ that had the ability to readily resist fungicides now suitable for a world in which fungi less able to resist are under attack.