Valley fever is found primarily in the hot, dry soils of the southwestern United States, northern Mexico, and scattered arid regions of Central and South America. Around 20,000 cases are reported in the U.S. each year, with the vast majority traced to Arizona, California, Nevada, and New Mexico. But the fungus has also been confirmed in parts of Washington state, and its range is expected to keep expanding.
Core Endemic Areas in the U.S.
The fungus that causes valley fever lives in soil and becomes airborne when that soil is disturbed. Historically, the four states with the most consistent local transmission are Arizona, California, Nevada, and New Mexico. Arizona and California account for the overwhelming majority of the roughly 21,000 cases reported to the CDC in 2023. Within those states, risk concentrates in specific counties where the soil chemistry and climate are ideal: alkaline, sandy ground in areas with hot summers and low annual rainfall.
In 2013, Washington state was officially added to the list. The fungus has been found in soils of south-central Washington, specifically in Benton, Kittitas, and Yakima counties. People and animals have likely been exposed in Yakima, Benton, Franklin, Walla Walla, and Spokane counties as well. Limited sampling has been done in other parts of the state, so the full extent of the fungus there is still unclear.
Where It Lives Outside the U.S.
Mexico’s northern border states carry the highest burden of valley fever in Latin America. Nuevo León, Tamaulipas, Chihuahua, Baja California, and Sonora all report significant numbers of cases. These regions share the same environmental profile as the U.S. Southwest: dry climate, alkaline soil, extreme summer heat (up to 50°C), and very low rainfall.
In Central America, confirmed locally acquired cases have been documented in Guatemala and Honduras. Guatemala’s Motagua River valley is a recognized hotspot, with skin-test surveys showing over 42% positivity in the area, meaning a large share of people living there have been exposed to the fungus at some point.
South America has its own scattered endemic zones. In Brazil, the northeast states of Ceará, Bahia, Piauí, and Maranhão have reported locally acquired infections since the late 1970s. Argentina has some of the best-studied pockets on the continent: Catamarca province shows infection rates around 40% in surveys, with neighboring areas in Córdoba, Santiago del Estero, and La Rioja also showing high exposure. The fungus also lives in parts of Colombia, Venezuela, western Paraguay, and southeastern Bolivia.
What Makes Soil Suitable for the Fungus
Valley fever isn’t found in just any dirt. The fungus thrives in alkaline, arid to semi-arid soils where temperatures typically range between 20°C and 40°C (roughly 68°F to 104°F). Growth drops off sharply at higher temperatures, especially when the soil is very dry. The pattern that feeds outbreaks follows what researchers call the “grow and blow” cycle: seasonal rain (during winter or monsoon seasons) provides enough moisture for the fungus to grow underground. Then, when hot, dry conditions return, the fungal filaments dry out and fragment into tiny spores, each only about 3 to 5 micrometers across. Those spores sit in the top layers of soil, waiting to be kicked up.
A single disturbed patch of contaminated soil can release millions of spores into the air. Under typical wind conditions, spore concentrations stay dangerous within about 1 to 2 kilometers of the source, though strong winds can carry them farther. The spores are spatially patchy, meaning you can be perfectly safe standing a short distance from someone who is breathing in a concentrated cloud.
When Cases Peak
Valley fever has a clear seasonal rhythm, at least in California, where it has been studied closely. Cases peak in September, October, and November, right at the tail end of the long dry season (April through October). Accounting for the time it takes to develop symptoms and get diagnosed, the actual period of highest exposure risk falls between July and September.
The weather pattern that produces the worst years is a wet winter followed by a dry summer. The rain fuels fungal growth underground, and the drought that follows dries the soil and breaks those fungal threads into airborne spores. Drought years themselves don’t necessarily spike cases, but the fall season after a drought ends tends to be especially bad. When rain finally returns to end a dry spell, the next year’s fall season often brings a pronounced surge in infections.
Who Faces the Most Exposure
Anyone who breathes in contaminated dust can get valley fever, but certain activities make exposure far more likely. The common thread is disturbing soil in endemic areas. Agricultural workers, construction crews, and people in mining or oil and gas extraction face elevated risk because their jobs involve digging, grading, or moving earth. Wildland firefighters, archaeological workers, and military personnel training in desert environments are also at higher risk. Even recreational activities like off-roading or hiking in dusty conditions can stir up enough soil to release spores.
A Range That’s Likely Growing
Valley fever’s geographic footprint is not fixed. Climate modeling projects that the endemic region will push northward into drier western states as temperatures rise. By the end of this century, under a high warming scenario, the area of suitable climate for the fungus could more than double, and the number of people who get sick could increase by 50%. States that could enter the endemic zone include Idaho, Wyoming, Montana, Nebraska, South Dakota, and North Dakota. Precipitation is expected to keep the fungus from spreading into wetter areas farther east or along the northern Pacific coast.
Washington state may be an early example of this expansion already underway. The fungus confirmed there in recent years is the same species found in California, and ongoing soil sampling continues to reveal new positive sites. For anyone living in or traveling to arid parts of the western U.S., Mexico, or South America, valley fever is a real and increasingly widespread risk tied directly to the dust beneath your feet.