In the winter of 2015, Dr. Tom Chiller received a call from public-health officials in Britain.

They were panicking. A new fungus was causing devastating infections at a large London hospital. “They weren’t able to get rid of it,” says Chiller, who was then chief of the fungal-diseases branch at the U.S. Centers for Disease Control and Prevention (CDC). “They didn’t know what to do.”The British officials said the fungus—a kind of yeast called Candida auris—appeared to be spreading from patient to patient. The officials said they were going to shut down parts of the hospital to curb its transmission. “That was just mind-boggling,” Chiller says.

At first he was more confused than alarmed. Candida is a kind of yeast that causes some of the most common fungal conditions, including thrush and vaginal yeast infections. Some species can cause severe illnesses, including bloodstream infections. But Chiller knew of no Candida species—indeed, no life-threatening fungus of any kind—that could spread so easily among people.

“I’ll never forget that call,” he says. “At first, I didn’t really believe it. Suddenly we have a yeast acting like a bacteria? It was weird.”First discovered by doctors in 2009 in the ear of a patient in Japan, Candida auris behaves differently from most other fungi. It can survive at higher temperatures and in saltier environments, making nooks like a human ear, armpit, and groin ideal places to grow. It is exceptionally sticky and clings doggedly to people’s skin and hair follicles, and to surfaces—including slick ones, like hospital floors and beds, that are designed to repel germs.

It is highly resistant to antifungal drugs, hard to kill with standard disinfectants, and can cause terrible, often deadly illnesses. Also, as British officials were starting to notice in 2015, it spreads easily through physical contact between people and via contaminated surfaces.

At the time of the call, though, much about Candida auris remained unknown. A few countries had reported a smattering of infections, but it hadn’t seemed like a big deal. No cases had yet been reported in the U.S. “It was a wake-up call,” Chiller says now. “We realized that, huh, maybe this little ear fungus isn’t just a little ear fungus anymore.”In the decade since that phone call, that realization has cemented. When about 200 people—mostly doctors and researchers—attended a three-day conference about fungal diseases near Denver in September, Candida auris was the first topic on the agenda.

To a nonscientist, the talks were dry, technical presentations on the pathogenesis and epidemiology of the disease, but to the attendees—and “almost everyone who works in mycology is here,” said Dr. Luis Ostrosky-Zeichner, an infectious-disease specialist at UTHealth Houston—they were red flags we should all be taking seriously.

Candida auris has rapidly gained ground since 2015. It has sickened tens of thousands of people in over 80 countries and six continents. The World Health Organization (WHO) has designated it one of the world’s highest-priority fungal threats. In the U.S., thousands of cases are now reported every year. The CDC said this summer that the fungus had already infected more than 3,000 people this year in 27 states. Up to 60% of people who are infected die.

“In some U.S. states, it’s just endemic now. It’s there to stay,” Chiller says.

Deadly fungal diseases were once considered very rare, but the proliferation of Candida auris points to a troubling trend: dangerous fungi are increasingly finding new opportunities to infect us, and human activity is spurring them on.

From the climate we’re warming to the medical treatments we use, we are creating conditions that allow fungal pathogens to thrive, infectious-disease experts say. Rising global temperatures, they say, likely led to the emergence of Candida auris and are driving the evolution of other fungal pathogens never seen before. Changing weather patterns are pushing some fungi to expand their geographic range, and the overuse of antifungals in medicine, agriculture, and livestock farming has driven them to develop resistance to these drugs.

“We are creating an environment that is favoring pathogenic fungi and drug resistance,” says Dr. Andrej Spec, a fungal-disease specialist at Washington University School of Medicine in St. Louis. “That means they are likely becoming better at infecting us, and also at evading our treatments. That’s a bad combination.”Fungi are ubiquitous organisms. They live in, on, and all around us. Every day, we inhale thousands—if not millions—of fungal spores.

There are an estimated 6 million species of fungi on the planet, but only about 300 of them cause diseases in people. “We are protected by our temperature and our immunity,” says Dr. Arturo Casadevall, chair of microbiology and immunology at the Johns Hopkins School of Public Health. The human body is too hot for most fungi to thrive, but as the earth warms, fungi are adapting.

Researchers think Candida auris, which likely originated in marine ecosystems, evolved a tolerance to salty conditions and high temperatures over time. The yeast can now survive at 107°F.

Candida auris is not the only threat finding hot new hosts. A new fungal pathogen is disfiguring cats and people in South America, and certain molds, which once infected only plants, are now sickening people too. Spec published a report in 2021 describing the first case of a person infected by a mold that had previously been known to damage only trees. Since then, two more people in the U.S. have been infected, Spec says. “It’s gone from never causing human disease in the history of medicine to three new cases of it in the last five years,” he says.

In October, the CDC said that cases of a different mold—a brain-attacking variety—have jumped fourfold in the past two years. “This is a nasty pathogen,” Spec says. It infects even healthy people and causes brain abscesses that in up to 70% of cases are fatal.

“We’re seeing a lot of these molds that were once rare pathogens becoming more common,” Spec says. “A lot of us are leaning toward the idea that this is probably being driven by climate change.”Spec and other doctors are working with public-health officials, patient advocates, and drug developers to try to stem the rise of threatening fungi. They’ve made some progress in recent years: raising awareness, doing more surveillance, and developing new drugs and tests, but the challenge is formidable.

As fungi develop mechanisms to better infect us, more people are becoming vulnerable to serious fungal diseases—because of advancements in medicine. Today, to fight autoimmune conditions and diseases like cancer, millions of people take drugs that suppress their immune systems. But immunocompromised people are at higher risk of contracting severe fungal infections. Thanks in part to those therapies, the growing population of immunocompromised people in the U.S. is now estimated to number at least 7 million.

“We have people who have immune conditions like multiple sclerosis, Crohn’s disease, and rheumatoid arthritis who might have died from these diseases in the past but aren’t dead thanks to these therapies. But they’re also immunosuppressed, so we have a bigger pool of people who are at risk,” Spec says.

More people are also being kept alive by invasive medical devices, such as heart-lung machines and ventilators, which can create pathways for germs including fungi to enter the body. “We’re making all this medical progress,” Casadevall says, “but that progress comes at a cost.”Ryan Zwick, a healthy 22-year-old, had just graduated from nursing school when COVID nearly struck him dead in December 2023. The virus, which research shows increases the risk of cardiovascular disease, severely damaged his heart.

Zwick spent a week in an intensive-care unit in Las Vegas before he was airlifted to a hospital in San Diego to receive more advanced care. He was intubated and sedated; his lungs, kidneys, and heart had all started to fail.

He was attached to a heart-lung machine, a life-support device that pumps a patient’s blood outside the body, via tubes placed into his sternum. “His chest was basically open for like a week,” his wife Emmy remembers. Candida auris took advantage.

When Zwick finally woke up more than a month after being admitted to the hospital, doctors told him he had a severe Candida auris infection. The yeast, they said, likely sneaked into his body through the tubes in his chest. “It got into my bloodstream and kept causing a lot of issues throughout my body,” he says.

He was treated aggressively with antifungal drugs, which kept the infection under control. But the fungus lingered.

Zwick was discharged from the hospital in April 2024, but he returned to the emergency room a couple of weeks later because blisters had appeared at the incision site on his chest. He soon learned that the Candida auris infection was causing osteomyelitis, a serious and painful condition in which bone tissue becomes inflamed. “They took out a bunch of bone and stuff, and put me on more antifungals,” he says.

More than two years later, Zwick, who works as a nurse in New Hampshire, continues to receive weekly infusions of a potent antifungal drug to manage his infection. His doctors have warned him that he will likely have to take them for life. “It’s basically chronic now,” says Zwick, who still sometimes has swelling and blistering at the incision site.

Zwick worries that available antifungal drugs could one day lose their sting. Candida auris is highly resistant to many antifungals; a growing number of infections are “pan-resistant,” which means no medicines are effective against them. “I just hope these drugs keep working for me,” Zwick says.

When Candida auris first emerged in the U.S. a decade ago, it was a very rare and often severe disease whose victims were usually sick, elderly, and immunocompromised people in hospitals and nursing homes. But the yeast has been expanding its reach: not just infecting more people, but a broader range of them, and causing a wider variety of illnesses too.

“Ten years ago, we’d maybe see one Candida auris patient every few months,” says UTHealth’s Ostrosky-Zeichner. “Now, we have several cases per day in Houston.”Texas, New York, Nevada, Florida, and California are among the U.S. states with the highest number of cases.

More young people, like Zwick, and people who aren’t immunocompromised are getting infected, doctors say. The yeast is also starting to cause new types of hard-to-treat infections. Spec says that this year, for the first time, he treated a woman with a vaginal yeast infection caused by Candida auris. She had diabetes that was well-controlled and was otherwise healthy, says Spec.

The woman had come to see him after getting recurrent yeast infections that hadn’t responded to standard treatments, he says. She recovered only after being treated for several days with an IV antifungal drug more powerful than what is usually used to treat such infections, Spec says.

Vaginal yeast infections are very common, with an estimated 3 in 4 women having at least one in their lifetime. A class of antifungals known as azoles are typically used to treat such infections, either as a cream, suppository, or oral pill, but Candida auris is more than 90% resistant to azole drugs. “It’s no joke when you have to tell a woman that she has a vaginal yeast infection for which you usually take one pill and move on, but in her case, she needs to get on IV medication,” Spec says.

Doctors say Candida auris is also emerging as the cause of another common disease: urinary tract infections (UTIs), which roughly half of women will experience at least once. Doctors have known that Candida auris can cause UTIs in people with catheters, but they are now seeing cases of people with tough-to-treat infections who don’t have any medical devices or major health complications.

Public-health experts worry that the fungus could become as widespread as pathogens like MRSA: methicillin-resistant Staphylococcus aureus, a superbug bacteria.

“Initially, we tried to stomp it out, but we realized early on that Candida auris was here to stay. It was going to be more like MRSA,” says Chiller. “MRSA used to be hospital-only, and now it’s on every skin boil on the high school wrestling team.”After Chiller got off the phone with his British counterparts in 2015, he went on a quest to better understand the fungus that he suspected would soon invade the U.S. He first wanted to know if Candida auris was already on U.S. soil but doctors had just missed it.

Under his direction, the CDC sent out an alert to hospitals and researchers, asking them to test samples of unidentified fungal pathogens in their repositories going back decades. “We didn’t find Candida auris historically,” he says.

The CDC also collaborated with public-health officials in parts of Asia, Africa, and South America, where Candida auris cases had been detected. Together, they studied samples of the fungus in an attempt to understand how it emerged. They discovered something remarkable: at least three distinct strains of Candida auris appeared nearly simultaneously, over the course of a few years, but separately—in different corners of the globe.

Many fungal-disease specialists and researchers now think climate change likely drove the yeast’s transformation from non-threatening environmental fungus to deadly pathogen. “It’s hard to explain the emergence of Candida auris simultaneously on all these continents and not include climate change as part of it. It just has to be. There’s no other good explanation,” says Dr. Peter Pappas, an infectious-disease doctor at the University of Alabama at Birmingham.

Fungi are sensitive to heat, but they are also very adaptable to rising temperatures. In laboratory experiments, researchers have put different fungi in incubators and, by steadily raising the temperature, have spawned fungi that are more tolerant to higher heat.

“It’s actually not that hard,” says Casadevall, who studies how fungi cause disease and was the first to publish the theory that global warming spurred Candida auris’ rise. “The slower you raise the temperature, the more the fungi can tolerate it. It doesn’t take a lot of mutations,” he says.

A study he published with colleagues last year showed how different microclimates within the same city can trigger changes in fungi. “We went to Baltimore and looked for fungi on the sidewalk in a cool part of town, where they had nice trees and it’s shaded, and then the same fungus in a warm part of town,” Casadevall says. “We found that even within one city, just two miles away, the fungi from the warm neighborhood grow at higher temperatures. So, it’s happening.”

Fungi are already a major threat to many plants, insects, and cold-blooded animals. Frogs everywhere are being wiped out by a fungal disease; rice and banana crops have been decimated by fungi; and millions of bats, which drop their body temperature when they hibernate, have recently died in North America because of a deadly fungus. Until now, however, most mammals have been protected by our high body temperatures.

The emergence of Candida auris is a “shot across the bow of humanity that these things can happen,” Casadevall says. “We can have an organism come out of nowhere, and begin causing disease.”Many other fungi are changing in ways that are worrying and, in some cases, dumbfounding doctors. As temperatures rise and rainfall patterns shift, some are spreading to new places. The soil-dwelling fungus Histoplasma, which had historically been confined in the U.S. to central and southern regions, has been steadily moving northward, eastward, and westward. U.S. cases have increased twofold in the past decade.

Valley fever, caused by inhaling microscopic fungal spores, is estimated to sicken up to 360,000 people in the U.S. yearly. The fungus was primarily found in the arid American Southwest until about a decade or two ago, when it began spreading to states like Washington, once considered too wet or cold for it.

Even fungal diseases considered mundane and easily treated, like ringworm, are evolving in alarming ways, doctors say. A new species of ringworm that can be resistant to all antifungal drugs has been spreading from the Indian subcontinent to the Middle East, Europe, and the U.S. in recent years. This new ringworm, which is highly contagious, causes chronic, inflamed, and itchy skin plaques all over the body.

“Think of athlete’s foot, but it’s now athlete’s body, and it’s untreatable,” Chiller says. He predicts this new disease, which experts think evolved in response to the overuse of over-the-counter antifungal medications, will become the predominant ringworm found globally in the next decade.

In Brazil and South America, outbreaks of a fungus that causes a disfiguring and sometimes fatal skin disease have been sickening cats, dogs, and people. Like Candida auris, the pathogen has mysterious origins. The fungus was first identified as a threat in 1998 after an outbreak in Rio de Janeiro. First, cats started showing up to the city’s clinics very sick and covered with lesions. Then, people and dogs did too. Infectious-disease doctors identified the culprit of these illnesses as a type of Sporothrix fungus. Infected cats were spreading the disease to dogs and humans through scratches, bites, and even sneezes.

That explanation didn’t seem to make any sense at first, says Dallas Smith, a fungal-diseases scientist at CDC who tracks emerging pathogens including Sporothrix.

A species of Sporothrix fungus called Sporothrix schenckii had long been known to doctors and scientists. The pathogen causes what is often referred to as “rose gardener’s disease” because the fungus, which lives in the environment, typically infects people who work with dirt and plants. “Gardeners would get pricked by a rose thorn, and that would provide the opening for the fungus to enter,” Smith says.

The fungus was known to infect cats, but that was exceedingly uncommon. The disease itself was also very rare. At the largest hospital for infectious diseases in Rio de Janeiro, the Evandro Chagas National Institute of Infectious Diseases, doctors typically saw two to three cases a year before 1998, says Dr. Dayvison Freitas, an infectious-dermatology researcher at the hospital.

But seemingly out of nowhere, something incited the emergence of a more virulent Sporothrix species spread by cats and not plants. It isn’t clear what pushed it to evolve to so effectively infect cats. “Is it because of climate change or environmental change or some other pressures that we don’t understand?”

Smith says. Sporothrix brasiliensis was named as a separate species in 2007. Infections caused by the fungus can result in multiple lesions in people, sometimes all over the body and face. It almost always leaves lasting scars, which are often dark and raised, says Freitas.

In up to 10% of cases, usually involving elderly or immunocompromised people, the fungus can enter the bloodstream and cause destructive illnesses. “The fungus can affect any organ or system, including the bones, joints, lungs, throat, brain. It can cause a kind of meningitis,” Freitas says. “If this happens, it is very, very severe, and the death rate is high.”Brazil has seen an explosion of the fungus in recent years. “Cats are spreading this disease among themselves, but also to humans at an alarmingly high rate,” Smith says.

From 1998 to 2001, fewer than 180 people, concentrated around Rio de Janeiro, fell sick with the disease. In recent years, thousands of people across the country are infected annually, Smith says.

The disease is now even infecting people in remote villages in the Amazon, Freitas says, and has emerged in other countries too. Cases of Sporothrix brasiliensis have been steadily sprouting up across South America in countries including Argentina, Paraguay, and Chile. Uruguay experienced its first outbreak of the disease in 2025, and Peru reported its first case this year, Smith says. “It’s just a matter of time before it ends up in the United States” and beyond, he says.

Sporothrix, says Chiller, is one of a growing number of fungi that are “dramatically changing our understanding of how we think fungi are supposed to act.”

At the Denver conference, a line of scientists followed Chiller wherever he went. Everyone there wanted to bend his ear. Chiller was decked out in his usual professional attire: a well-cut jacket with a colorful, bespoke fungi-print lining and a matching pocket square. (Chiller, whose grandfather was a tailor, has about 15 fungi-themed custom jackets.)“In every generation, there tends to be a few people who are a force multiplier,” says Spec. “They change the field. In the case of mycology, the success that exists is often due to Tom Chiller.”

Chiller has been working for decades to draw the world’s attention to fungi’s gathering threat. He was the deputy director of the CDC’s fungal-diseases branch for about 10 years and its chief for another decade. Under his leadership, the department doubled in size to about 45 people and expanded its work to include surveillance of priority fungal diseases, including Candida auris, and research into emerging pathogens, as well as new treatments and diagnostic tools.

He was instrumental in the development and widespread use of a cheap, simple, and accurate test, using just a pinprick of blood, for Cryptococcus—an environmental fungus that can cause meningitis. “It’s definitely saved tens of thousands of lives,” Chiller says. “I’m very proud of that.”

Chiller left the CDC last year after 25 years at the agency, months after Robert F. Kennedy Jr. was named head of the U.S. Department of Health and Human Services. Chiller says he had been growing increasingly disenchanted at the agency since COVID. “During the pandemic, it was turmoil for everybody. CDC took a big hit reputationally,” he says. Then Kennedy took the helm, and Chiller’s team was cut by about a quarter, he says. “We had to prioritize what we could do.”

Chiller says he felt “handcuffed” by restrictions imposed on CDC staff in the spring of 2025. “They cut all our travel, they canceled all meetings. We didn’t really have budgets,” he recalls. “I thought to myself, If this is the future, it’s not for me.”Chiller is now chief medical officer at IMMY, a company that develops tests for fungal diseases. He’s also a board member of the nonprofit Global Action Fund for Fungal Infections and of MyCARE, a fungal-disease patient-advocacy organization. Now, he gets to do what he loves most: advocating for fungal-disease awareness.

In September, he attended a roundtable discussion about fungal diseases with Kennedy and other U.S. health officials. Chiller, who helped coordinate the meeting, says it was the first time in his career that he had been in a room with a U.S. Health Secretary to talk only about fungus. That felt like progress, he says.

Chiller knows just how critical it is for health leaders and professionals to be familiar with fungi. “Think Fungus” was the catchphrase of the CDC’s first Fungal Disease Awareness Week, which Chiller helped launch in 2017. Doctors often miss fungal diseases because they fail to think of them, research shows.

Though the spread of fungi in a warming world may seem like an impossible foe, there is hope in the fact that diagnosing fungal infections early makes it much more likely that a patient will survive—and researchers, including Chiller, are working to help patients take advantage of that edge.

They’ve got a long way to go: a 2019 paper showed that almost half of some 270 patients in Arizona with valley fever—a fungal disease Chiller studied as a young doctor—weren’t diagnosed correctly for a month or more.

Justin John lived that nightmare. The 27-year-old software engineer from Arizona was wrongly diagnosed with COVID, the common cold, and irritable bowel syndrome before doctors realized—more than six months later—that he was sick with valley fever.

There were “lesions literally everywhere inside my body,” he says. After lumps caused by the fungus started appearing on his forehead and neck, doctors discovered that the fungus had eaten holes through parts of his spine and skull. He underwent two surgeries to insert a metal plate into his forehead, and replace a part of his vertebrae with a metal disc. “The vertebra was so dissolved, they couldn’t save any of it,” he says.

John says he was healthy, active, and had no underlying medical conditions before he got sick. “It’s in the dirt. You can get it anywhere at any time,” he says. “I could have gotten it when I was walking to Walmart.”

Blood tests are currently used to diagnose valley fever, but false negative results are common, causing many missed diagnoses. IMMY is developing a new rapid urine test for the disease that it says will be easier to administer and more accurate. “So many patients, even here in Arizona where this disease is endemic, get missed,” says Dr. Fariba Donovan, a valley fever specialist at the University of Arizona who treated John. “This test could really decrease that delay in diagnosis.”These are steps in the right direction, but Chiller says we need to take greater strides if we want to win in the race against fungi.

Many decades ago, after encountering very sick and dying valley fever patients, Chiller says he “learned a lot about how devastating and complex fungal diseases can be.”He’s still waiting for the rest of us to catch up.