California and federal public health officials say valley fever, a potentially lethal but often misdiagnosed disease infecting more and more people around the nation, has been on the rise as warming climates and drought have kicked up the dust that spreads it.

The fever has hit California's agricultural heartland particularly hard in recent years, with incidence dramatically increasing in 2010 and 2011. The disease — which is prevalent in arid regions of the United States, Mexico, Central and South America — can be contracted by simply breathing in fungus-laced spores from dust disturbed by wind as well as human or animal activity.

The fungus is sensitive to environmental changes, experts say, and a hotter, drier climate has increased dust carrying the spores.

"Research has shown that when soil is dry and it is windy, more spores are likely to become airborne in endemic areas," said Dr. Gil Chavez, Deputy Director of the Center for Infectious Diseases at the California Department of Public Health.

Longstanding concerns about valley fever were heightened last week when a federal health official ordered the transfer of more than 3,000 exceptionally vulnerable inmates from two San Joaquin Valley prisons where several dozen have died of the disease in recent years. A day later, state officials began investigating an outbreak in February that sickened 28 workers at two solar power plants under construction in San Luis Obispo County.

Although millions of residents in Central California face the threat of valley fever, experts say people who work in dusty fields or construction sites are most at risk, as are certain ethnic groups and those with weak immune systems. Newcomers and visitors passing through the region may also be more susceptible.

Nationwide, the number of valley fever cases rose by more than 850 percent from 1998 through 2011, according to the Centers for Disease Control and Prevention. In 2011, there were more than 20,400, with most cases reported in California and Arizona.

In California, according to the CDC, valley fever cases rose from about 700 in 1998 to more than 5,500 cases reported in 2011. The disease has seen the sharpest rise in Kern County, followed by Kings and Fresno counties.

Out of the 18,776 California cases between 2001 and 2008, 265 people died, according to the state health department.

Arizona saw an even steeper rise: The number of reported cases there went from 1,400 in 1998 to 16,400 in 2011, with the highest rates of infection occurring in Maricopa, Pima and Pinal counties.

Drought periods can have an especially potent impact on valley fever if they follow periods of rain, said Prof. John Galgiani, director of the Valley Fever Center for Excellence at the University of Arizona. Rainfall leads to fungus bloom, but limits dust.

"When it dries up, that's when the fungus goes into the air," Galgiani said. "So when there is rain a year or two earlier, that creates more cases if drought follows."

Another reason for the increase in cases, Galgiani said, is new residents, who are more susceptible to the disease, relocating to areas with the spores.

In addition, the CDC and the California Department of Public Health say improved reporting methods and better diagnosis also partially explain the increase in valley fever.

Despite that, an estimated 150,000 valley fever infections go undiagnosed every year, the CDC says. That's because valley fever is difficult to detect and there's little awareness of the disease, experts say. The fever often causes mild to severe flu-like symptoms, and in about half the infections, the fungus — called Coccidioides — results in no symptoms.

But in a small percent of cases, the infection can spread from the lungs to the brain, bones, skin, even eyes, leading to blindness, skin abscesses, lung failure, even death.

"Valley fever is a very common problem here, and it devastates people's lives," said Dr. Royce Johnson, professor of medicine at UCLA and chief of infectious diseases at Kern Medical Center. "But many patients don't know about it, and some physicians are only vaguely aware of it because half of our physicians come from out of state."

Dale Pulde, a motorcycle mechanic in Los Angeles County, said he contracted the disease three years ago after traveling to Bakersfield in Kern County and was coughing so hard he was blacking out; he spit blood and couldn't catch his breath. For two months, doctors tested him for everything from tuberculosis to cancer until blood tests confirmed he had the fever.

After two lung operations, Pulde gave up his job and is on disability. He says he has to take anti-fungal medication that costs him more than $2,000 per month out of pocket. He had to sell his house in Sylmar, Calif., to raise money for his treatment.

"When I found out that health officials knew about (this disease) and how common it is, I was beside myself," said Pulde, now 63. "Why don't they tell people?"

California public health officials say they are working to educate and train the public and doctors to recognize the illness.

The state has trained county health departments about the fungus, Chavez said. It has also included information on valley fever in a newsletter the California Medical Board sends to the state's licensed physicians. The CDPH website and social media feature information and data about the disease, including advice to limit outdoor activities on dry, windy days.

As prison officials gear up to move inmates from the endemic areas, doctors and patients say more needs to be done, including funding research to work on a cure.

"If the state is so concerned about prisoners, they should be worrying about all of us who live and work in the valley," said Kathy Uhley, a former realtor from Los Banos who contracted the fever last year.

-----Contact Gosia Wozniacka via Twitter at (at)GosiaWozniacka