Arturo Casadevall, professor of medicine, first encountered the specter of fungal disease as an intern and resident at Bellevue Medical Center in New York City in the mid-’80s. It was the height of the HIV/AIDS epidemic, and the city’s hospitals groaned under the strain.

What killed AIDS patients was not the virus itself but rather a constellation of opportunistic infections that their shattered immune systems could no longer fight. In 1980, the U.S. logged 500 cases of a disease called cryptococcus. By 1985, New York City alone clocked several thousand cases of this fungal disease that can cause lung infections and meningitis. As Casadevall made his rounds each morning, he found it was commonly fungal infections that had landed his patients in the hospital.

“If I wanted to work on what was killing people, it wasn’t the HIV but the fungi. At the time, these diseases seemed to come out of nowhere,” he says.

For Casadevall, it was the start of a careerlong fascination with these organisms that share our planet. In What if Fungi Win?, co-authored with Stephanie Desmon, he takes a deep dive into the fungi that share our planet — both species that cause disease as well as those organisms that perform valuable, life-sustaining functions. Because as much as Casadevall has seen the suffering caused by fungal infections for most of his life, he also knows that we depend on fungi to break down waste, help produce bread and beer, and maintain stable ecosystems.

“The benefits we get from fungi far, far exceed any of the problems they cause. Without them, life on Earth as we know it would not be possible,”
he says.

Casadevall hypothesizes that we might owe our existence to fungi. In the wake of the asteroid strike 66 million years ago that led to the end of the dinosaurs, the global climate cooled. For fungi, which struggle to replicate at high temperatures, this was a tremendous opportunity, he notes. Cold-blooded lizards provided an ideal home. Warm-blooded mammals, however, were simply too hot for the fungi to survive and replicate. A world overrun with fungal disease could have provided an evolutionary opening to mammals, Casadevall says, though he points out that the idea is speculation and can’t truly be tested.

Although antiretroviral medications have transformed HIV from a death sentence to a manageable chronic illness, the threat of fungal infections hasn’t diminished. If anything, the threat is greater than ever, he notes in his book. Our current climate is undergoing rapid change, and it’s this global warming that has Casadevall most concerned. Rising planetary temperatures may force fungi to evolve ways to replicate more efficiently at warmer temperatures, thus unravelling some of the protections afforded us by our fast metabolisms and high body temperatures. He points to the new pathogen Candida auris, which appeared simultaneously in South America, Africa and Europe a decade ago and has now spread globally.

“They came out of the environment with resistance to two of the three types of antifungal drugs. It’s a problem that’s getting worse,” he says. “And there are no vaccines to this or any fungi.”

Despite decades of awareness of the harm caused by fungal pathogens, Casadevall’s book is surprisingly bullish on our continued coexistence. Scientists are turning to fungi to create new materials without the pollution of plastics and as laboratory workhorses for synthetic biology. Just as our history has been shaped by fungi, Casadevall says, so, too, will our future.

“A future world where fungi win might not be a bad one. They might have already won — we just don’t see it,” he says.