Valley Fever is on the rise in California, according to preliminary data from the state department of public health.
Cases have increased annually over the past three years, with higher numbers during the first part of 2025 compared to the same period in 2024.
Valley Fever is a fungal disease that can cause respiratory symptoms and, in some people, severe pulmonary infection and chronic lung damage. California cases were previously concentrated in the central part of the state, but have been spreading statewide as weather conditions become hotter and drier.
“Valley Fever is an urgent health crisis because of climate change,” says UCLA Health clinical microbiologist Shaun Yang, PhD.
Also known as coccidioidomycosis, or cocci, Valley Fever is caused by a dimorphic fungus – one that exists as both a mold and a yeast, Dr. Yang explains.
It starts out as a mold found in the soils of the desert Southwest. The mold produces infectious spores, which can be carried by dust and stirred up by agriculture and construction when the weather is dry. When people inhale spores in dust, the spores germinate. Once inside the human body, they cease to grow as mold and instead become a pathogenic yeast.
Risk to lungs and other organs
Many people infected with Valley Fever are asymptomatic, Dr. Yang says, though some experience such symptoms as cough, fever, chest pain and tiredness. People can also develop severe pulmonary infection, which can be chronic and difficult to treat.
The illness can also spread outside the lungs to infect other organs, including the brain. The mortality rate is high when the infection spreads beyond the lungs, says Manish Butte, MD, PhD. Dr. Butte received an $8.4 million grant from the National Institute of Allergy and Infectious Diseases in 2022 to improve diagnosis and treatment of Valley Fever by learning what can go wrong with the immune system’s lines of defense.
“California probably spends around a billion dollars a year taking care of patients with Valley Fever and disseminated disease,” says Dr. Butte, the E. Richard Stiehm Professor of Pediatric Allergy, Immunology and Rheumatology at the David Geffen School of Medicine at UCLA. “But the treatments today resemble those developed in the 1990s, and we still don’t have a good idea which patients will get really sick and which ones will have milder disease.”
People with compromised immune systems – including those who have received organ transplants, people with HIV and people receiving chemotherapy – are at greater risk of developing disseminated disease, or disease that spreads throughout the body, Dr. Butte says. Pregnant people are also at higher risk of poor outcomes from Valley Fever, Dr. Yang adds.
Still, it’s not clear why some people, even those who were previously healthy and not immunocompromised, become very ill or develop disseminated disease from Valley Fever.
“It’s not always that if you are very healthy, you will not get severe disease,” Dr. Yang says. “We have yet to find out what exactly is the host factor that can lead to more severe infection.”
Increased spread is likely
Repeated drought seasons punctuated by occasional rainy winters contribute to the spread of Valley Fever. Wet winters encourage mold and spore growth while dry summers make it easier for those spores to float through the air, where they can be unwittingly inhaled.
It’s unclear whether the Los Angeles wildfires could contribute to the spread of Valley Fever in the area, Dr. Yang says. The U.S. Centers for Disease Control and Prevention reported a cocci outbreak among wildland firefighters in 2021.
“Even though we are not entirely sure if fires can lead to increased cases, it is prudent to be cautious and avoid inhaling dust in the fire areas,” Dr. Yang says.
Hotter, drier conditions are causing this fungus to spread northward from areas where it’s traditionally found – Central California, Arizona, New Mexico and Texas.
“Maybe in a few decades, half of the United States will be endemic for cocci,” he says. “Because of climate change and global warming, this endemic fungus will become more widespread and cause more infections.”
It’s difficult to avoid breathing the spores in areas where the fungus is endemic, according to the CDC. It advises staying inside and closing windows during dust storms and avoiding dusty areas, such as construction or excavation sites. Indoor air filtration can also be helpful.
Most people who are infected with Valley Fever are asymptomatic or have mild symptoms that resolve with no treatment or with antifungal medication.
