In the midst of their revelry, festival-goers in California unwittingly kicked up the invisible spores of a dangerous fungus while dancing in the dust.

Floating in the air, the soil pathogen surreptitiously slipped into the lungs of at least five attendees at the Buena Vista "Lightning in a Bottle" music festival.

Three individuals had such severe symptoms, they were hospitalized with 'Valley fever' – a disease endemic to the southern San Joaquin Valley that is caused by two species of Coccidioides fungi, which grow in the soil and dirt as mold.

The California Department of Public Health (CDPH) fears that more cases from the festival are going undiagnosed.

The festival, which was attended by more than 20,000 people, took place at Buena Vista Lake in Kern County in May.

While most people who breathe in spores from this fungus do not develop Valley fever, in rare cases the pathogen can infect the lungs, leading to fatigue or fever, making it difficult to breathe, and causing some patients to cough up blood.

Most people with Valley fever, known as coccidioidomycosis or cocci, get better without treatment, but in severe cases, symptoms can linger for months or years. The infection can sometimes even spread from the lungs to other parts of the body. With no known cure, there is a risk of paralysis or death.

In 2019, 190 Californians were diagnosed with a cocci infection in their skin, bones, or brain, which significantly increases the risk of mortality.

"Past outbreaks of Valley fever have been associated with exposure to dust and dirt at outdoor events and job sites where dirt was being disturbed in areas of California where Valley fever is common," write officials at the CDPH.

Luckily, Valley fever cannot spread from one person to another, but even though the disease is not contagious, between 1998 and 2015, the number of people falling ill from the Coccidioides fungus increased by 400 percent.

In California alone, from 2000 to 2022, reported Valley fever cases surged by 800 percent.

In 2017, the US National Oceanic and Atmospheric Administration (NOAA) published a study that linked these rising cases to a doubling of dust storms in the southwest.

"I cannot think of any other infection that is so closely entwined with climate change," University of California immunologist Rasha Kuran told The Washington Post in 2023.

In 2024, a month after the "Lightning in a Bottle" music festival, Kern County experienced its driest June in the past 130 years. This came on the back of massive historic rainstorms, which caused widespread flooding in the region.

Such a wet winter, experts explain, can encourage the growth of mold and its spores. Then, when the weather dries out, these spores are unlocked from the soggy soil and kicked up into the air with the dust.

The event at Kern County in May ran over Memorial Day weekend, the perfect time for cocci spores to spread.

In some videos from the festival on social media, crowds can be seen covered in what appears to be a haze of dust. One account on TikTok described the event as a "dusty magical journey".

Festival goers were warned of high winds and dust storms, but not of the invisible threat that the fine particles can deliver to the lungs.

"California probably spends around a billion dollars a year taking care of patients with Valley Fever and disseminated disease," says infectious disease specialist Manish Butte from the University of California, Los Angeles.

"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."

Some researchers fear that more droughts and floods in the future could make Valley fever endemic to nearly the whole west coast. About a decade ago, Washington state got its first case – a long march north from the San Joaquin Valley where the disease got its name.

The outbreak of Valley fever at the "Lightning in a Bottle" music festival is an omen of illnesses to come.