The scientific term for the disease is coccidioidomycosis, most commonly known as valley fever. Whatever its name, the fungus-caused illness—which can be fatal—is being called a silent epidemic.

“This is a high priority for the medical community,” said Dr. Tom Chiller, a fungal expert with the Centers for Disease Control, told CNBC by phone.

“More people are getting exposed to it, and it’s an increasing problem we want to stop,” added Chiller.

The fungus that causes valley fever lives in soil in the U.S. desert Southwest and parts of Mexico, Central America and South America. Inhaling the fungus’ airborne spores can cause flu-like symptoms that can turn into pneumonia, meningitis or even worse.

Though valley fever is not contagious, cases have been on the rise. Less than 5,000 cases were reported in 1995. That number had risen to more than 20,000 by 2011. And the CDC estimates that some 150,000 cases go undiagnosed annually.

In the U.S., over 70 percent of cases occur in Arizona and 25 percent in California.

The CDC says that about 60 percent of the people who inhale the spore do not contract valley fever. But one out of 200 who do get it will develop the form that can be fatal, according to a study conducted by the University of Arizona. An estimated 30 to 40 Arizona residents die from valley fever each year.

Valley fever is not new. Studies about the illness date back to the 1950s, and patients in the Southwest have been diagnosed with it for decades.

The symptoms of valley fever are similar to common illnesses—fever, cough, headache, a rash, muscle aches and joint pain—which can delay proper treatment, said Chiller at the CDC.

There’s no vaccine or cure, but the CDC plans to conduct clinical trials on two common treatments. Most remedies involve taking antifungal medication.

Though anyone can get it, valley fever is most common among older adults, African-Americans, Asians, women in the third trimester of pregnancy and people with weakened immune symptoms.

Reasons for the rise in the incident rate include population increases where the fungus lives, construction (which churns up the soil and exposes the organism) and heightened awareness, Chiller said.

Like other illnesses, valley fever has costs.

According to the CDC, nearly 75 percent of valley fever victims miss work or school for about two weeks. More than 40 percent of valley fever patients are hospitalized, with the average cost of a hospital stay for treatment reaching almost $50,000.

“It takes a huge toll on daily lives,” Chiller said. “The symptoms can last for weeks or months and even longer.”

Valley fever cases have been diagnosed in Washington state, beyond its normal geographical range, he added. That’s cause for concern, as is the increasing number of cases among younger patients.

“We don’t know how it travels. It could be windblown or even from rodents,” Chiller said. “But it’s clear we need to take this seriously and try to find a vaccine. Right now, we just need to raise awareness so people can get treated before they get sicker.”

—By CNBC’s Mark Koba