Toe nail fungus (onychomycosis), is a condition that destroys the toe nail. It can be caused by different types of fungi which thrive in the dark & moist environment inside shoes. Toenail fungi feed on keratin, the protein that makes up the toenail surface. Toenail fungus generally belongs to a group of fungi called dermatophytes, which include Trichophyton rubrum. Less common causes of onychomycosis or toe nail fungus include yeasts and molds.  

"What does Trichophyton rubrum do? Simply put, it causes athlete's foot, the perennial curse of locker rooms, and it's also a major player in jock itch, that tormenting fungal infection in a young athlete's groin…In persons middle-aged and older, whether former athletes or not, T. rubrum tends to infect - and ultimately to destroy - the toenails….In the United States more than 90% of all toenail fungus infections are caused by T. rubrum."

Source: "The War Against Toenail Fungus" by Dwight Thomas

Millions of Americans see a podiatrist for treatment of toe nail fungus, but a lot more don't seek help.

When a toenail develops a fungal infection, it typically turns yellow and becomes thick and overgrown.

Toe nail fungus is usually a chronic condition that gradually worsens to involve more and more of the toenail. If the nail comes off, the new toenail that grows usually is infected with fungus also.