Pathogens in soil are a danger to firefighters, but smoke may transport spores that cause valley fever and other infections into cities too.

GLOWING RED SKIES and bitter ash-laden air are increasingly common across much of the US as the warming climate fuels vast wildfires. For years now, researchers have understood that wildfire smoke, and the noxious gases and soot particles it carries, isn’t merely an unpleasant experience that forces people to shut windows and herd children indoors. It’s a significant health hazard that not only triggers asthma and breathing problems, but can harm immune systems for years.

But new research is revealing that the threat is more complex than previously known—and also more dangerous, if that’s possible. The churning convection in those enormous plumes can scoop up pathogens from disturbed soil and charred vegetation and carry them long distances from the fire lines. Many of those microbes are still living, and potentially capable of causing infections. Among them, researchers are most worried about fungi, which cause slow-growing internal infections that are difficult to diagnose and treat, and can be disabling and even deadly. That includes the fungal infection valley fever, which lurks in dry Western soils, causes a serious illness resembling pneumonia, and is spreading north as the climate warms.

Thanks to drones carrying sampling equipment, there is already evidence that smoke plumes are packed with viable bacteria and fungi. One early study has linked a major 2017 California fire with increasing numbers of fungal infections in hospital patients up to 200 miles away. Now several research projects are attempting to precisely identify the microbial contents of smoke and to correlate infection records with maps of the directions in which smoke drifts during fire seasons.

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