Even the most fastidious people may develop fungal infections of the toenails, and the most common cause is spread of athlete’s foot to the skin at the end of the nail where the fungus then invades the nail bed.

If left untreated, nail fungus, known clinically as distal subungual onychomycosis, can infiltrate the nail itself, eventually causing the nail to separate from the nail bed. Prior to that point; however, people typically notice their nail becoming white or yellow and thickening. Debris may form under the nail, and the nail itself may begin to crumble.

“Fungal infections of the nail are notoriously difficult to treat because of the protective nature of the nail over the underlying nail bed. Additionally, nails grow slowly and have very little blood supply,” says Dr. Amy Musiek, a dermatologist with Washington University Physicians. “Initially, the lesions are usually asymptomatic, but over time the nails may become painful. Most people notice the changes in the nail appearance prior to the occurrence of pain.”

While toenails are most often infected due to the proximity of athlete’s foot, fingernails may succumb to fungus, particularly when synthetic nails are used. Sterility of instruments at nail salons and proper nail maintenance are important to help avoid such problems.

“It’s all about the moisture content. It promotes fungal growth,” says Dr. Richard Wittock, a podiatrist and president of the Ankle and Foot Institute. “Sweat and moisture contribute to that.”

He notes that some types of fungal organisms have a predilection for nails or skin. “Those organisms are commonly on the ground or on towels that haven’t been washed in while. If those spores get into the nail plate or between the toes, they can start a fungal infection of the skin or the nail plate,” he says. “Another common situation would be in a nail that’s been polished for too long where that superficial fungus can start under the nail polish.” He adds that the fungal spores can be shed and transferred to other people in the household.

People who attempt to treat a fungal infection with over-the-counter topical medications may have little success. These drugs may not penetrate the nail and eradicate the fungus from all affected areas.

“The most effective treatments are the oral agents terbinafine and itraconazole,” Musiek says. “They have cure rates of 70 to 90 percent. Ciclopirox nail lacquer can be used in mild disease when only one or two nails are involved, but the cure rates are less than 20 percent with this product. There are ongoing studies with photodynamic therapy and laser devices,” she says. If not treated successfully, surgical removal of the nail may be necessary.

Prevention involves good hygiene, keeping the feet dry, wearing shower shoes in public facilities, keeping nails properly trimmed, and wearing breathable shoes and moisture-wicking socks.