Back at the turn of the 21st century, valley fever was an obscure fungal disease in the United States, with fewer than 3,000 reported cases a year, mostly in California and Arizona. Two decades later, cases of valley fever have exploded, increasing roughly sevenfold by 2019.
And valley fever isn’t alone. Fungal diseases in general are appearing in places they have never been seen before, and previously harmless or mildly harmful fungi are becoming more dangerous for people. One likely reason for this worsening fungal situation, scientists say, is climate change. Shifts in temperature and rainfall patterns are expanding where disease-causing fungi occur; climate-triggered calamities can help fungi disperse and reach more people; and warmer temperatures create opportunities for fungi to evolve into more dangerous agents of disease.
For a long time, fungi have been a neglected group of pathogens. By the late 1990s, researchers were already warning that climate change would make bacterial, viral, and parasite-caused infectious diseases such as cholera, dengue, and malaria more widespread. “But people were not focused at all on the fungi,” says Arturo Casadevall, a microbiologist and an immunologist at the Johns Hopkins Bloomberg School of Public Health. That’s because, until recently, fungi have caused humans relatively little trouble.
Our high body temperature helps explain why. Many fungi grow best at about 12 to 30 degrees Celsius (roughly 54 to 86 degrees Fahrenheit). So though they find it easy to infect trees, crops, amphibians, fish, reptiles, and insects—organisms that do not maintain consistently high internal body temperatures—fungi usually don’t thrive inside the warm bodies of mammals, Casadevall wrote in an overview of immunity to invasive fungal diseases in the 2022 Annual Review of Immunology. Among the few fungi that do infect humans, some dangerous ones, such as species of Cryptococcus, Penicillium, and Aspergillus, have historically been reported more in tropical and subtropical regions than in cooler ones. This, too, suggests that climate may limit their reach.
Today, however, the planet’s warming climate may be helping some fungal pathogens spread to new areas. Take valley fever, for instance. The disease can cause flu-like symptoms in people who breathe in the microscopic spores of the fungus Coccidioides. The climatic conditions favoring valley fever may occur in 217 counties of 12 U.S. states today, according to a 2019 study by Morgan Gorris, an Earth-system scientist at the Los Alamos National Laboratory, in New Mexico.
But when Gorris modeled where the fungi could live in the future, the results were sobering. By 2100, in a scenario where greenhouse-gas emissions continue unabated, rising temperatures would allow Coccidioides to spread northward to 476 counties in 17 states. What was once thought to be a disease mostly restricted to the southwestern U.S. could expand as far as the U.S.-Canadian border in response to climate change, Gorris says. That was a real “wow moment,” she adds, because that would put millions more people at risk.
Some other fungal diseases of humans are also on the move, such as histoplasmosis and blastomycosis. Both, like valley fever, are seen more and more outside what was thought to be their historical range.