Fungus is everywhere—in the carpet, in locker room showers, in your shoes. Fungal infections of the nails (onychomycosis) can be a persistent and sometimes embarrassing problem, and occasionally can become painful. These infections develop when fungi, known as dermatophytes, burrow under nails causing them to become crusty, thick, discolored and distorted. It’s not only embarrassing, but it can cause complications for people with diabetes or immune disorders. Plus, it’s really hard to treat. An estimated 13 percent of Americans, or roughly 35 million people, have toenail fungus, says James Christina, DPM, director of scientific affairs at the American Podiatric Medical Association.