Enlarge / Histoplasma capsulatum is a species of parasitic, yeast-like dimorphic fungus that can, if inhaled, cause a type of lung infection called histoplasmosis. (credit: Nanoclustering/Science Photo Library)

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 per year, mostly in California and Arizona. Two decades later, cases of Valley fever are exploding, increasing more than sevenfold and expanding to other states.

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 turning deadly 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 early 2000s, researchers were already warning that climate change would make bacterial and viral infectious diseases like cholera and dengue more widespread. “But people were not focused at all on the fungi,” says Arturo Casadevall, a microbiologist and immunologist at the Johns Hopkins Bloomberg School of Public Health. That’s because, until recently, fungi haven’t troubled humans much.