For years, Vance Vredenburg, a blond, Chaco-wearing frog biologist at San Francisco State University, watched miserably as his favorite animals, mountain yellow-legged frogs, fell victim to chytrid. "These frogs were incredibly abundant for a very long time," he says. "Now they've become so scarce that most people don't know they live up there."

Vredenburg has monitored chytrid's march through the Sierra Nevada since the early 2000s, often hiking 15 miles over 12,000-foot passes to check alpine lakes for the disease. He knew chytrid had arrived when mountain yellow-legged frogs disappeared from lakes or floated belly-up near shore. He watched some frogs, too sick to swim, drown. "It was one of the worst experiences of my life," he says.

On a 2002 trip to Sixty Lake Basin in Kings Canyon National Park, he found the usual carnage. But at a remote lake in Yosemite National Park that same year, a few populations of mountain yellow-legged frogs appeared to be living with the usually fatal disease. What was the difference?

He found the first clues in 2005, during a talk by Reid Harris, the biologist who discovered the chytrid-inhibiting bacterium that would also inspire McKenzie's research. Just like McKenzie, Vredenburg began to suspect that microbes helped determine which frogs lived or died. To test the theory, he and one of Harris' post-docs hiked back into the Sierras. They caught and swabbed mountain yellow-legged frogs in a Kings Canyon lake that was still chytrid-free, and returned to sample the surviving Yosemite frogs.

Their results affirmed Vredenburg's hypothesis. A significantly higher proportion of the Yosemite frogs had Janthinobacterium lividum on their skin – the same anti-fungal species Harris found on salamanders. Just one year later, the Kings Canyon population went extinct. Harris, Vrendenburg and others then exposed one group of mountain yellow-legged frogs to chytrid, and one to J. lividum and chytrid. The results were dramatic: Every frog in the first group died. Every frog in the second group lived.

The experiment changed Vredenburg's career. "It forced me to become a much bigger-picture biologist," he says. Instead of doing field research on "my favorite little frog that nobody else in the world cares about," he now works in a lab with bacteria and has graduate students studying the skin microbes of amphibians around the world. He hopes to discover a way to fight chytrid by treating frogs with higher concentrations of anti-fungal bacteria than already live on their skin.

One obvious way to do that is to dose them all with J. lividum. But scientists are hesitant to spread the microbe around for fear of inadvertently spawning a new biological threat, as happened with the chytrid fungus. When animals are exposed to microbes their immune systems have never encountered, "it's a huge concern," explains McKenzie. Plus, J. lividum may not take to every amphibian, she says. "It's not one-size-fits-all."

Scientists prefer to identify and culture anti-fungal bacteria already familiar to the frogs. But it's hard to do. Even though DNA analysis revealed local strains of J. lividum growing on McKenzie's endangered boreal toads, she has failed to grow any of them in a petri dish. That means she can't test how reliably the bacteria can protect toads from chytrid in the lab. And so her ultimate goal – to increase the survival rate of captive toads released into the wild to boost populations by bathing them in beneficial bacteria – remains out of reach.

Scientists worry that because research takes so long and chytrid kills so quickly, amphibians will continue to go extinct before they find the bacteria that could save them. To get ahead of the fungus, Harris and a grad student have set their sights on Madagascar, one of the world's last chytrid-free areas. They hope to find and stockpile native fungus-fighters before chytrid appears. That way, if the disease arrives, they can respond quickly by releasing local strains of bacteria into the environment and letting them spread from frog to frog. "I like to call it crisis biology," Vredenburg says. "We don't have time to figure out the nuances. We have to do something now before these frogs go extinct."