In February, a dermatologist in New York City contacted the state’s health department about two female patients, ages 28 and 47, who were not related but suffered from the same troubling problem. They had ringworm, a scaly, crusty, disfiguring rash covering large portions of their bodies. Ringworm sounds like a parasite, but it is caused by a fungus—and in both cases, the fungus was a species that had never been recorded in the US. It was also severely drug-resistant, requiring treatment with several types of antifungals for weeks. There was no indication where the patients might have acquired the infections; the older woman had visited Bangladesh the previous summer, but the younger one, who was pregnant and hadn’t traveled, must have picked it up in the city.

That seemed alarming—but in one of the largest and most mobile cities on the planet, weird medical things happen. The state reported the cases to the Centers for Disease Control and Prevention, and the New York doctors and some CDC staff wrote up an account for the CDC’s weekly journal.

Then, in March, some of those same CDC investigators reported that a fungus they had been tracking—Candida auris, an extremely drug-resistant yeast that invades health care facilities and kills two-thirds of the people infected with it—had risen to more than 10,000 cases since it was identified in the US in 2016, tripling in just two years. In April, the Michigan Department of Health and Human Services rushed to investigate cases of a fungal infection called blastomycosis centered on a paper mill, an outbreak that would grow to 118 people, the largest ever recorded. And in May, US and Mexican health authorities jointly rang an alarm over cases of meningitis, caused by the fungus Fusarium solani, which seemed to have spread to more than 150 clinic patients via contaminated anesthesia products. By mid-August, 12 people had died.

All of those outbreaks are different: in size, in pathogen, in location, and the people they affected. But what links them is that they were all caused by fungi—and to the small cadre of researchers who keep track of such things, that is worrisome. The experts share a sense, supported by incomplete data but also backed by hunch, that serious fungal infections are occurring more frequently, affecting more people, and also are becoming harder to treat.

“We don’t have good surveillance for fungal infections,” admits Tom Chiller, an infectious disease physician and chief of the CDC’s mycotic diseases branch. “So it’s hard to give a fully data-driven answer. But the feeling is definitely that there is an increase.”

The question is: Why? There may be multiple answers. More people are living longer with chronic illnesses, and their impaired immune systems make them vulnerable. But the problem isn’t only that fungal illnesses are more frequent; it is also that new pathogens are emerging and existing ones are claiming new territory. When experts try to imagine what could exert such widespread influence, they land on the possibility that the problem is climate change.