Valley Fever explodes in California: New hotspots see 300% increase

Kern County, California – California is on track for another record-breaking year of Valley fever cases, with more than 3,100 infections already confirmed in 2025 — nearly double the number seen at the same time in 2023. Health experts say climate change, shifting weather patterns, and increased exposure to disturbed soil due to wildfires, droughts, and construction fuel the surge.

Valley fever, or coccidioidomycosis, is caused by inhaling spores of the Coccidioides fungus. The fungus grows in dry, dusty soil throughout the western U.S., particularly in California and Arizona. Once rare outside the Southwest, the disease is increasingly diagnosed in new areas as the climate becomes more arid and soil conditions favorable to the fungus expand eastward.

“There is no question that the number of cases of coccidioidomycosis is enormously higher than before,” said Dr. Royce Johnson, director of the Valley Fever Institute at Kern Medical. “If you want to see me, you’d have to wait until July — and that goes for my colleagues, too.”

According to state data, California saw up to 9,000 cases annually between 2017 and 2023, but preliminary numbers from 2024 show a dramatic jump to over 12,600 cases — a historic high. The sharp rise is being driven not just by endemic areas like Kern County, which has already reported more than 900 cases this year, but also by emerging hot spots in Monterey, San Luis Obispo, Merced, and Contra Costa counties.

Gail Sondermeyer Cooksey, an epidemiologist with the California Department of Public Health, noted that Valley fever appears to be spreading beyond traditional zones. “It appears to be spreading out,” she said. “Cases in Contra Costa have tripled since last year.”

Environmental factors, especially climate variability, are believed to play a significant role. A 2022 study in The Lancet Planetary Health found that while drought years suppress infections, wet winters followed by hot, dry summers create ideal conditions for the fungus to spread. This pattern has become more frequent in California recently, intensifying disease risk.

Wildfires may also be contributing. Research from 2023 found higher infection rates following large fires in areas with high Valley fever transmission. Fires stir up soil, releasing fungal spores into the air, while post-fire reconstruction activities continue to disturb contaminated ground.

“We warned first responders to January’s Los Angeles County fires about the risk,” said Sondermeyer Cooksey. “Anything that disturbs the dirt can exacerbate Valley fever.”

Diagnosing Valley fever is difficult because its symptoms — fatigue, fever, cough, headache, and joint pain — resemble other illnesses like flu or COVID-19. In severe cases, the infection can spread to the brain or bones, becoming life-threatening. Though only 1% of cases result in complications, the fungus never entirely leaves the body, and suppressed immunity can trigger relapses years later.

Festivals have also emerged as unexpected vectors. At least 19 people who attended Lightning in a Bottle in Kern County last year were diagnosed with Valley fever; eight were hospitalized. The event is being held there again this month.

Experts warn that as California’s climate shifts, so will the terrain of infectious diseases. “As climate change alters the timing and intensity of precipitation in California,” said Simon Camponuri, a PhD candidate at UC Berkeley, “we see longer high-risk periods for Coccidioides exposure.”