The scientific term for a fungal infection of the nails is onychomycosis. These infections are caused by a type of microscopic fungus known as dermatophytes. This family includes some closely related fungi, such as those which cause ringworm and athlete’s foot. Like most kinds of fungus, toenail fungus thrives in a place which is moist, warm and dark; so the inside of a shoe is one of the best places for them to grow!
Onychomycosis, aka Tinea Unguium, is a chronic fungal nail infection caused by dermatophytes (including the genera Trichophyton, Epidermophyton and Microsporum), the most common pathogens of onychomycosis, with Trichophyton rubrum and Trichophyton mentagrophytes accounting for over 90% of onychomycoses. Many of the remaining cases are caused by nondermatophyte molds (e.g. Aspergillus, Scopulariopsis, Fusarium).
Dermatophytes get under the nail fold at the end of the nail and grow underneath the nail. When they take hold, the nail become thick, yellowish-brown or darker in color, deformed and foul smelling. Debris may collect beneath the nail plate, white marks frequently appear on the nail plate, and the infection is capable of spreading to other toenails, the skin, or even the fingernails.
Fungal infections are highly contagious, so an important part of the treatment of toenail fungus is prevention of reinfection.