Mike Trout, the Angels’ top prospect and number three on FanGraphs’ Top 100 Prospects list, hasn’t seen any game action in spring training. Since the start of spring camp, Trout’s been suffering from severe flu-like symptoms, lost ten pounds and been too weak to play in games. He says doctors have ruled out Valley Fever and are treating him with anti-viral medication.

But the fact that Trout had to answer questions about Valley Fever shows how much it has seeped into the baseball reporter’s lexicon. Just a few weeks ago, we learned that Mets first baseman Ike Davis has a mild case of Valley Fever. And, of course, there’s former Diamondbacks outfielder Connor Jackson, who contracted a severe form of the illness in 2009, leading to pneumonia. Jackson missed a good portion of the 2009 season, and has never regained the form or power he showed with Arizona.

What is Valley Fever and why are professional baseball players coming down with it?

Valley Fever is a fungal infection caused by coccidioides spores found in the soil in the desert southwest. The fungi are stirred into the air by activities that disrupt the soil, like farming, construction and wind storms. In its mildest form, Valley Fever symptoms include fever, cough, chest pain, night sweats, chills, headache, fatigue, joint aches, and a red spotty rash. More severe symptoms, like those experienced by Connor Jackson, include pneumonia and meningitis. The course of the disease varies but it can take from six months to a year for a patient to fully recover.

According to the Valley Fever Center, a collaborative project of St. Joseph’s Hospital and the University of Arizona College of Medicine, more than 100,000 Arizona residents contract Valley Fever each year, a dramatic increase over the last several years. About 30,000-40,000 develop symptoms severe enough to seek medical attention, but because the disease presents like flu, many patients are misdiagnosed.

Most of the Valley Fever cases in Arizona come from Maricopa County, home to Chase Field, where the Diamondbacks play their regular-season home games, and all ten Cactus League spring-training ballparks. Five of the spring-training complexes are also used in the Arizona Fall League. Many major-league players also have their offseason home in Arizona.

That’s a lot of major and minor leaguers in the Phoenix area playing and practicing baseball outdoors. And while baseball isn’t considered a risk factor like farming or construction, the more time one spends outdoors, the higher the risk of inhaling the coccidioides spores. The Mayo Clinic generally recommends that those with the highest risk of exposure wear masks, wet the soil around them, and stay inside during dust storms.

We contacted several teams with spring-training facilities in Arizona to see if they were taking any special precautions to lower the risk for players. None were willing to discuss Valley Fever with us.

Although Valley Fever doesn’t appear to present a significant health risk to players who live or play in Arizona, with two reported cases among major leaguers in the last three years, MLB should take steps to educate teams and players about the disease and how to lower the risk of infection.