You get valley fever by breathing in fungal spores from contaminated soil or dust. The fungus lives in dry, arid ground across the southwestern United States and parts of Mexico and Central America. When that soil is disturbed, microscopic spores become airborne and can travel deep into your lungs with a single breath. Over 21,000 cases were reported in the U.S. in 2023 alone.
The Fungus Lives in Soil
Valley fever is caused by a fungus that grows in the top few inches of desert soil. It thrives in hot, dry conditions and lies dormant during periods of drought. When rain comes, the fungus grows a network of filaments underground. As the soil dries out again, those filaments break apart into tiny spores, light enough to float on the slightest breeze.
Any activity that breaks up the ground can launch these spores into the air. You don’t need to be digging a trench or plowing a field. Something as simple as driving on a dirt road, hiking across open desert, or even standing near a construction site can expose you. Dust storms are especially effective at spreading spores over long distances, sometimes miles from the original source.
Where It Happens
The fungus is concentrated in the southwestern U.S., particularly Arizona and California, which together account for the vast majority of reported cases. Nevada, New Mexico, and Utah also see regular infections. In 2013, Washington state was added to the list after locally acquired cases were confirmed there.
Outside the U.S., the fungus is found in parts of Mexico and Central America. Climate projections suggest the range is expanding northward as temperatures rise. Research published through the American Meteorological Society estimates that under high warming scenarios, the number of counties where the fungus can survive could more than double by 2090, spreading from 217 counties to 471 and reaching as far as the U.S.-Canada border. The number of states considered endemic could grow from 12 to 19.
Activities That Put You at Risk
Certain jobs carry a significantly higher risk because they involve constant soil disturbance in endemic areas. According to OSHA, workers in construction, roadwork, and agriculture face the greatest exposure. Digging with heavy equipment, grading land, and even moving livestock across dry pastures all generate the kind of dust that carries spores. Military personnel training in desert environments, wildland firefighters, and archaeological field workers are also at elevated risk.
You don’t need to work outdoors to be exposed, though. Recreational activities count too. Off-roading, mountain biking, gardening, and camping in endemic areas can all stir up enough soil. Even being inside during a major dust storm can lead to exposure if the building isn’t well sealed.
What Happens After You Breathe in Spores
Once inhaled, the spores settle into your lungs and transform into larger, round structures that reproduce by releasing hundreds of new cells into the surrounding tissue. Your immune system mounts a response, and in many cases that response is enough to contain the infection entirely.
About 60% of people who inhale the spores never develop symptoms at all. Their immune system handles the fungus quietly, and they may never know they were infected. The remaining 40% typically start feeling sick one to three weeks after exposure. Symptoms usually look a lot like a respiratory infection: fatigue, cough, fever, shortness of breath, and sometimes chest pain. Some people develop a rash on their legs or torso. Because it mimics the flu or pneumonia, valley fever is frequently misdiagnosed, which can delay appropriate treatment.
In a small percentage of cases, the infection spreads beyond the lungs to the skin, bones, joints, or the membranes surrounding the brain. This disseminated form is more serious and requires long-term antifungal treatment. People with weakened immune systems, pregnant women (particularly in the third trimester), and people of African or Filipino descent face a higher risk of the infection becoming severe.
It Is Not Contagious
Valley fever does not spread from person to person or from animals to people. You cannot catch it by being near someone who is sick, sharing a household, or touching an infected wound. In extremely rare cases, infection has occurred through organ transplantation when the donor had valley fever, but this is not a typical route of transmission. The only meaningful way to get it is by inhaling spores from the environment.
Reducing Your Exposure
There is no vaccine for valley fever, and because the spores are microscopic and widespread in endemic soil, completely avoiding exposure in affected areas is difficult. Still, practical steps can lower your risk. If you live in or travel to the Southwest, try to stay indoors during dust storms and keep car windows closed on unpaved roads. Wetting soil before digging can reduce the amount of dust that becomes airborne.
For outdoor workers in endemic zones, wearing an N95 respirator during soil-disturbing tasks provides a meaningful layer of protection. Working upwind of excavation sites, using enclosed cabs on heavy equipment, and suppressing dust with water are all standard recommendations from OSHA. If you’re new to an endemic area, be aware that your risk is highest in the first year or two. Long-term residents who were previously infected often develop some degree of immunity.
Timing matters as well. Cases tend to peak in late summer and fall, when hot, dry, and windy conditions are most common. If you can schedule outdoor projects during cooler, wetter months, your exposure risk drops.