What life-threatening illness can you get from repotting plants, attending a rodeo or going spelunking? If you didn't guess histoplasmosis, you're not alone.

This week's Morbidity and Mortality Weekly Report, chronicle of all things infectious, reports on the surprising appearance of histoplasmosis, a lung infection caused by a fungus, in four people in Montana.

The fungus in question, Histoplasma capsulatum, is common in the Midwest and Midatlantic, according to the researchers at the Centers for Disease Control and Prevention and in Montana who penned the report. That was news to us. So we talked with John Bennett, chief of the clinical mycology section at the National Institutes of Health, to get up to speed.

"There are huge areas of the country where this is relatively common, including here in Washington, D.C.," said Bennett, who wasn't involved in the study. "The thing that made this unusual is that Montana is outside the usual area."

Uh, exactly how common in Washington? This is suddenly starting to strike uncomfortably close to home.

Skin tests have found that a sizable number of people in the Mid-Atlantic have been infected with histoplasmosis, Bennett says. When infected you might get a cough or feel a bit fluish. But most people shake it off and never get really sick.

Indeed, histoplasmosis and other fungal infections typically are lethal only in people whose immune systems are weak. The four people in the MMWR report had other health issues that could have made them more vulnerable, ranging from chemotherapy for colon cancer to mononucleosis.

The four people who fell ill in Montana in 2012 and 2013 ranged from a 17-year-old boy who liked caving, camping and had worked as a landscaper to a 79-year-old retired rancher. The boy had had mono; the rancher had colon cancer. All recovered from their infections, though some were sick for months with pneumonia and other health problems.

Histoplasmosis, sometimes called histo, is spread by the droppings of birds and bats. It's common in soil, so common that AIDS patients and other people with compromised immune systems are warned to have someone else repot the plants.

It's possible that birds and bats are spreading the fungus from South Dakota and North Dakota, where it's been known for years. Or it could be that some of these people had been exposed years earlier.

"The problem about this fungal infection, you can get infected now and not get sick until years later," Bennett says. "The older gentleman, did he really get it in Montana? Or did he get it somewhere else and it reactivates 20 years later?"

Only one of the patients, the retired rancher, had a confirmed case of histoplasmosis, Bennett cautions. So this may not signal a looming fungal invasion of Montana.

But Dr. Henry Masur, chief of critical care at the NIH Clinical Center and an infectious disease researcher, says he wouldn't be surprised to see histo and other infectious diseases cropping up where you wouldn't expect to see them.

"We're a more and more mobile society, both in terms of people and in terms of pathogens," Masur told Shots.

OK, so you don't live in Montana, or the Midwest or in D.C. Think you don't have to contend with fungus? Think again.

Farmers and others in the Midwest have to contend with the fungus Blastomyces dermatitidis, which also lives in soil.

And the Pacific Northwest has been dealing with a particularly dangerous strain of Cryptococcus gatti for several years now. It has caused at least 40 deaths.

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