Fungus Among Us

The effects of a changing climate in the form of flooding and fires are widely discussed in insurance circles. But other risks come with warming temperatures, some that experts say need more attention. The spread of fungal diseases that kill humans and devastate crops is one of those.

Fungal diseases are not new, nor are concerns about their expansion. Since the 1960s, the practice of growing fewer crops and crop types on larger parcels has generated genetically consistent staples, cereals, and root crops that are particularly vulnerable to rapid fungi infection, the Swiss Re Institute said in its 2025 SONAR report on emerging risks.

But the threat appears to be changing alongside the global climate. Fungal infections largely cannot survive the body temperature of a healthy human. But as fungi adapt to climatic changes, they become more resilient at higher temperatures. As one expert put it during a May 2025 symposium at Duke University: “The thermal barrier is narrowing.” (Many will recognize this as the fictional jumping-off point for the fungal apocalypse in the video game and television show The Last of Us.)

The changing climate is also said to be affecting the geographic spread of fungal pathogens in various ways, including creating more regions in which the temperatures are warm enough for them to grow and changing growth and reproduction cycles.

The use of fungicides is a further factor in the risk fungal infections pose. In 2024, University of Manchester fungal investigator David Denning estimated that fungi diseases kill 3.8 million people globally each year. That is nearly double the projected figure that Denning and colleagues published over a decade earlier. He cited a number of contributing factors, including increasing resistance to antifungal drugs due to overuse of select fungicides on crops—a threat recognized by the World Health Organization (WHO), among other bodies.

“The excessive use of fungicides in agriculture have [sic] resulted in the spread of more virulent strains to livestock and humans. There are already cases of multi-drug resistant fungi limiting medical treatment options for those affected,” the SONAR report notes. “While currently still on small scale, the development of anti-fungal resistance to drugs could contribute to higher mortality rates, perhaps more so during natural disasters and in the midst of other disease outbreaks.”

Fungal Risk

Fungi are eukaryotic organisms that come in single-cell (yeast) and multicell forms, often found in soil and other moist environments (but also your refrigerator if you don’t eat those leftovers). Common examples include mushrooms, truffles, and mold. The uses of fungi are broad and significant, from pizza topping to medicinal, but their dangers are equally real— infections of the lungs, brain, eyes, bones, and skin, to start, through means such as inhalation of spores or contact with an open wound.

The estimated 3.8 million deaths each year from fungal diseases would represent almost 7% of all deaths, Denning wrote in research published in the July 2024 issue of The Lancet Infectious Diseases.

Fungi already threaten food security on the international level. “Plant-pathogenic fungi account for around 20% crop yield destruction, with an additional 10% loss post-harvest,” researchers wrote in a 2024 article in the journal npj Antimicrobials and Resistance.

In the United States alone, the U.S. Centers for Disease Control and Prevention (CDC) currently projects that fungal diseases each year will cause 7,288 deaths, 133,555 hospitalizations, and 13.3 million outpatient visits to primary care, urgent care, and other providers. Annual direct medical costs are estimated at $13.4 billion, plus $1.7 billion in work time lost and $4.3 billion in death costs, says Dr. Jeremy Gold, an epidemiologist and fungal expert in the CDC’s Mycotic Diseases Branch.

The actual expenses are probably significantly higher, Gold notes, as the CDC study did not account for costs that are hard to quantify, including medical treatment and care before the fungal disease diagnosis, long-term care, non-prescription drugs, and pain, psychosocial distress, and other effects on families.

“These estimates are higher than previous estimates; however, previous estimates were calculated using different methods and cannot be directly compared,” according to Gold. “CDC continues to investigate these trends; however we do not currently have available estimates for how future changes might affect human health.”

Pathogens Adapting to Higher Temperatures

No vaccines exist for fungal infections. Otherwise healthy humans have generally been protected from serious harm by the fact that fungi, for the most part, cannot survive at mammals’ body temperature. But that could change.

Just a few hundred of the roughly 144,000 identified species of fungi have been found to infect humans; and few of those are infectious to those who do not have compromised immune systems due to conditions such as influenza, AIDS, and asthma. However, as fungi adapt to survive increased environmental heat from climate change, additional species may also become dangerous to humans due to that narrowing between ambient and bodily temperatures, researchers wrote in a May 2024 article in the journal PLOS Pathogens. That barrier already drops in the human body by about 0.03 degrees Celsius each decade after birth.

“There are probably thousands of species with human pathogenic potential that currently do not cause disease because they can’t tolerate human temperatures,” Arturo Casadevall, chair of molecular microbiology and immunology at the John Hopkins Bloomberg School of Public Health, tells Leader’s Edge. “Predicting the species that will break through human temperature defenses is very difficult, if not impossible, since [there] is an element of chance in all of this.”

While this threat has not been proven, the fungi “most likely to make this jump” are those that are known to cause diseases in humans but do not handle body temperature well, says Asiya Gusa, assistant professor of molecular genetics and microbiology at the Duke University School of Medicine. She cites Fusarium oxysporum, which is better known for causing plant diseases, and Cryptococcus deneoformans, which can cause invasive infections in humans though generally is limited to the skin.

At least one fungal pathogen, multi-drug-resistant yeast Candida auris, is already believed to have developed due to climate change adaptation, according to the PLOS Pathogens article. Three separate clades (organisms with a shared ancestor) were found to have emerged on three continents between 2012 and 2015. Global warming might have played a role in this development, as the yeast is able to grow at elevated temperatures relative to comparable fungi.

“This suggests that its acquisition of thermal tolerance and consequent transition from environmental fungus to human pathogen may have been relatively recent,” the researchers wrote. “Concern is growing that other fungi may similarly adapt.”

Candida auris can cause various infections, including potentially deadly infection of the bloodstream, the CDC says. It spreads particularly in medical facilities and is difficult to defeat with antifungal drugs. It has been identified in the United States only since 2016 but already spreads expansively—as of early August, nearly 3,000 infections in 21 states had been reported this year, Nextstar reported.

The fungus is one of 19 that the World Health Organization first listed in 2022 as posing the highest risk to public health.

“We can’t quantitate it but the emergence of Candida auris is a cautionary tale,” Casadevall says. But it’s not the lone threat.

Heat stress has also been connected in research studies with sped-up genetic mutation of the fungal pathogen Cryptococcus deneoformans, which can cause fatal lung infection. Genetic mutation could promote increased heat tolerance, virulence, and resistance to medications, the PLOS authors wrote, though they said additional research is needed in this area.

Changing Climates Change Geographic Spread

There is also the potential for fungal infections that have historically been restricted to tropical and subtropical regions to expand geographically as the regions suitable for their growth change, the PLOS Pathogens article reports. Researchers cited the fungi that cause the lung infection valley fever (coccidioidomycosis), which historically has been found in hot, dry growth zones in the U.S. Southwest and sectors of Central and South America. But at least a handful of apparently locally acquired cases were identified from 2014 to 2017 in Washington state. Temperature and rainfall modeling suggest that the fungi could reach Minnesota in this century.

There is also evidence of histoplasma fungal infections moving beyond their traditional U.S. home around the Ohio and Mississippi River valleys, possibly due to climate change, research suggests. The spore-based infection can damage the liver, spleen, and other organs. “We simply don’t have data for many other disease-causing fungi so we can’t track changes in incidence,” Gusa says. “The lack of surveillance and diagnostic tools to track fungal diseases is a major problem that thwarts our ability to understand emerging fungal health risks.”

Flooding, hurricanes, and other extreme weather can also promote fungal diseases. For example, spores carried by strong winds from a severe convective storm could infect a human body through a wound sustained in the same event.

The risks posed by geographic spread of fungal pathogens aren’t limited to human infection. Fungal pathogens already undermine global crop output, with straightforward repercussions on reduced yield, along with more indirect social, environmental, and economic impacts, researchers wrote in a 2023 article in the journal Environmental Research. They threaten millions who make their living in agriculture.

“Changing climate conditions change the opportunity for fungal pathogens outbreaks to occur at various times and places, changing the threats farmers face and the management regimes that are necessary to control them,” the article states. “It is therefore critical to model future changes in fungal pathogen burdens to ensure food security in the future.”

According to the authors, the geographic spread in a region of crops at risk for specific pathogens is likely to change as temperatures rise. As noted in the case of human infections, the increasingly warm world could also produce pathogen strains that adapt better to higher temperatures. Pathogen inoculum (fungi and other biological agents that initiate infection) are showing a growing capacity to survive the warmer winter conditions outside of the tropics, heightening the potential for outbreaks.

Warmer temperatures also extend growing seasons, promoting more rapid growth and reproduction of certain fungal pathogens. There might be more than one disease cycle in one growing season.

Environmental changes can also affect phenology (the timing of life cycle events for plants and animals), overwintering, stability, and migration. “As a result of global warming, the infecting pathogen may spread within the host, disperse within the host, and/ or horizontally to new hosts,” according to the Environmental Research article.

Jon West, a plant pathologist at Rothamsted Research in the United Kingdom, notes an example of this this year when wheat began to flower in the United Kingdom in late May. This occurred weeks earlier than it would have in years past, as the U.K. experienced a dry and warm spring in 2025. “And similarly the fungi are releasing their spores a little bit earlier as well due to some of the same environmental cues”—effectively remaining in sync with crop growth, West says.

Plant disease requires a vulnerable host, efficient inoculation by the pathogen, and the right environmental setting, according to West and colleagues. Among environmental factors, weather has the greatest impact on plant diseases and how disease extends over a given area, they noted.

“Fungi need a damp surface or a wetness film or at least very, very high humidity for the spores to germinate, grow a little bit along a surface,” West tells Leader’s Edge. “At that point they’re very vulnerable. If it dries out, they could dry up before the pathogen manages to get into the plant. So it needs wetness for a certain period of time.”

Extreme weather can raise the threat of fungal diseases in crops in ways not dissimilar to its potential impact on human infections. “When you get these very, very unusual storms with strong winds and heavy rain, you can get such a fine spray generated and such mobile air that the spray with the spores in them can actually go now miles, tens of kilometers,” West says. “So it’s moving pathogen populations around a little bit more than we’re used to seeing.”

Antifungal Resistance

For humans at risk of a fungal infection, there are only three primary classes of antifungal drugs for systemic infection: azoles, echinocandins, and andpolyenes. Candida auris and other pathogenic strains have demonstrated resistance to at least one of those classes on an international scale, according to the PLOS Pathogens authors, who say this development is already a “global health threat.” Previous research has shown a connection between use of agricultural fungicides and development of medication-resistant infections treated with drugs that are chemically similar to those fungicides.

“Resistance to antifungal medicines is partly driven by inappropriate antifungal use across the One Health spectrum,” the World Health Organization stated in 2022 as it issued its fungal priority pathogens list. “For example, injudicious use of antifungals in agriculture was linked to the rising rates of azole-resistant Aspergillus fumigatus infections.”

That could point to increasing incidence of this human peril as extreme weather causes agricultural productivity losses that lead to even more applications of fungicide, PLOS Pathogens warns.

Fungal diseases already can cause upward of $200 million in annual crop losses, according to the Agricultural Research Service within the U.S. Department of Agriculture. Some of the most vulnerable crops are also key to human sustenance, including wheat, rice, and maize.

Excessive application of fungicides could result in accumulation of the agents in soil and water, in turn producing environmental impairment liability and workers compensation claims linked to workers sickened by drug-resistant strains of fungi, Swiss Re adds.

Multiple leading U.S. crop insurers did not respond to queries or said they could not comment regarding the potential risks posed to their branch of the industry by climate change and overuse of fungicides.

Disease Management

Curbing the expansion of fungal diseases begins with tracking new and multi-drug-resistant outbreaks of crop and human diseases, the Swiss Re Institute said in the SONAR report. “This should include the impacted targets and the identification of pathways by which fungi distribute.”

The World Health Organization’s fungal priority pathogens list, which ranks the 19 pathogens based on their threat level to public health, can point the way for focusing surveillance and research resources, Swiss Re said. In the report, WHO urges nations to enhance diagnostic capabilities for managing fungal infections and to commit to disease monitoring.

“There is a need for sustainable investments in research, development, and innovation. More investments are needed in basic mycology research, R&D of antifungal medicines and diagnostics. Innovative approaches are needed to optimize and standardize the use of current diagnostic modalities globally,” the 2022 document says. “In addition, public health interventions are needed to highlight the importance of fungal infections, including through incorporating fungal diseases and priority pathogens in medical (clinical) and public health training programmes and curricula at all levels of training. Similarly, collaboration across sectors is required to address the impact of antifungal use on resistance across the One Health spectrum.”

Experts interviewed for this article concurred on the need for additional research, along with stepped-up surveillance of disease events. Asaya calls for the federal government to fund research on fungal adaptation to environmental changes, including heat adaptation and drug resistance. Federal funding is also needed to prepare new antifungal drugs, she says.

The likely optimal solution for addressing the fungicide-antifungal connection would be to establish distinct classes of antifungal drugs for agriculture and medicine, to prevent one sector from promoting treatment resistance in the other, Casadevall says. “However, such a goal would require a major investment of resources by government, pharmaceutical companies and the agricultural industry, combined with political will to make it happen.”

He expresses optimism, though, that greater public awareness will drive increased investment in studying fungal biology in order to develop disease countermeasures and learn more about fungal threats.

The Swiss Re Institute further urged containment measures to prevent the spread of fungal pathogens. That could involve border measures for goods and humans, for some fungi.

Training is needed to prepare agriculture, construction, and healthcare personnel to quickly identify and respond appropriately to indicators of fungal infection, the report says. Significant losses result when cases of fungal infection are detected late or are never detected.

Biodiversity and application of particular plants or crops serve as a non-chemical substitute for fungicides to deter infection in the agriculture sector, according to SONAR.

Users must ensure careful application of any new tools to prevent development of additional multi-drug-resistant fungi, which will require risk management in each application area, it adds.

“In their risk assessment, insurers should remain cognizant that, in a world where international collaboration is declining and state budgets are shrinking, a global response to fungal spread may not be forthcoming and at the very least be inadequate,” the Swiss Re Institute said. “And in this environment, it could be that insurers need to shoulder a higher claims load.”