If you notice your toenail looks yellow, has white marks on it, or contains debris underneath, chances are a fungus is hanging out under your nail. "It's a highly contagious infection that's transmitted from foot to foot, particularly in germy hotbeds like nail salons and gyms," says Carolyn McAloon, DPM, spokeswoman for the American Podiatric Medical Association. Usually it's not painful, although wearing a shoe that's pressing on the infected area can irritate it, she says.
Fungi thrive in dark, wet, warm areas. To prevent one from taking hold, wear shoes made of breathable fabrics, like leather or canvas, and be sure to take off your socks and shoes to air out your feet after exercising or working all day. A fungal nail can be treated by trimming and filing any loose areas of the nail, soaking feet in water that contains a few drops of tea tree oil (which is a natural antifungal), using a topical antifungal treatment (if necessary) and bleaching the shower to kill any lingering fungus, says Dr. McAloon.