With winter approaching, seasonal illnesses like the flu are already surging. There is another to add to the long list of illnesses to look out for – Valley fever.

Cases of Valley fever are spiking across the southwest. Infections in Arizona have jumped 45% from what they were this time last year, an uptick of almost 4,000 cases. California cases have increased by about 600 since last year, according to the state’s health department.

While most cases are mild, a fraction of patients develop severe disease and die. Signs and symptoms are difficult to spot and there’s no vaccine to prevent it, although researchers at the University of Arizona are developing one.

Here’s a primer on Valley fever and why there’s been a rise in cases.

What is Valley fever? Signs and symptoms

Valley fever, also known by its clinical name coccidioidomycosis, is a lung infection that is rarely fatal and typically goes away without treatment.

You can contract it if you breathe in fungal spores carried by dirt and dust. The fungus, called coccidioides, releases spores when dirt and dust are disturbed. An infection can occur when a person inhales these spores.

Hot and dry conditions are favorable to the fungus that causes the infection. The fungus grows across the Southwest and is prevalent in regions of southern California, but Valley fever cases have been reported as far north as Washington state, according to Dr. John Galgiani, the director of the University of Arizona Valley Fever Center for Excellence.

The good news is the infection is not contagious: a person must be exposed to fungal spores to contract it.

As with other illnesses that spike in the fall and winter, fever and cough are among the signs you might have Valley fever. Other symptoms include fatigue, shortness of breath, muscle aches or joint pain and headaches, all of which are similar to symptoms of other illnesses, making diagnosis especially tricky, Galgiani said.

Valley fever diagnosis and treatment

Symptoms of Valley fever show up about one to three weeks after a person inhales the spores.

Diagnosis requires a blood test that’s not included in routine lab work when a person goes to the emergency room or urgent care, Galgiani said. Immediate testing and diagnosis can prevent wasted time trying drugs that are ineffective for Valley fever.

“Being tested is not a bad idea if you’re sick,” he said, especially if you have a protracted or severe flu or symptoms of pneumonia.

Most people get better on their own, Galgiani said. But chronic infections can occur in people with weakened immune systems and lead to pneumonia.

Knowing the cause of pneumonia-like symptoms can make or break treatment for Valley fever. Pneumonia-like symptoms are typically treated with antibacterial or antiviral drugs, but those medicines are ineffective against Valley fever because it’s a fungal infection.

For people with a severe infection, treatment can include taking an oral antifungal medication for three to six months, according to the Centers for Disease Control and Prevention.

The most severe infection, called disseminated coccidioidomycosis, happens when the fungus spreads beyond the lungs to other parts of the body, including the skin, bones and brain. Disseminated coccidiomycosis occurs in up to 2% of cases.

Fluctuating environmental patterns cause year-to-year differences

Studies show variable weather caused by climate change could be contributing to the rise and spread of fungal infections.

The fungus that causes Valley fever thrives in hot and dry climates – which is typical in central and southern Arizona. Phoenix sits at the center of the endemic region for the fungus, and Maricopa County, which surrounds it, accounts for half of U.S. cases, according to Galgiani.

With Phoenix experiencing the hottest summer on record, the heat and lack of measurable precipitation this year created the perfect storm for the fungus to thrive.

“The modeling for climate change is that it will, over this century, get to the Canadian border,” Galgiani said. “We’ll see if that holds true, but the model would be that the endemic region will expand across the western United States.”

Coccidioides spores can be disrupted through wind, dust storms and other activities that stir up dust and dirt. Even hiking and horseback riding can disturb the ground enough to release the microscopic spores.

A vaccine is on the horizon

Galgiani’s team recently developed the first vaccine to prevent Valley fever. Although it’s designed to be used on dogs, researchers hope to engineer a human version soon.

Around 6 to 10% of dogs in Maricopa, Pima and Pinal counties in Arizona contract Valley fever each year, according to the Valley Fever Center for Excellence. Dogs inhale the spores just like humans. The infection cannot spread from dogs to humans.

Researchers are hopeful the federal government will approve the shot by next year, which would allow veterinarians to begin administering it to dogs and other animals they think may benefit from it. Cats and zoo animals can also catch Valley fever, according to the center Galgiani directs.

The University of Arizona research team received a grant from the National Institutes of Health to use the dog vaccine to help engineer a human version, which Galgiani says is necessary as the endemic region continues to grow.

“If there becomes more fungus in the soil and more spores get in the air, there can be five times as many cases every year,” he said. “It’ll make the idea of having a vaccine that much more appealing.”