Picture of Ringworm of the Nails
Ringworm is a fungal infection. When it affects the nails it is called onychomycosis or tinea unguium. Fungal nail infection makes nails thick, brittle, and change colors. Affected nails may be white, yellow, or brown.
Certain medical conditions that affect circulation increase the risk of onychomycosis. Diabetes, peripheral vascular disease, and obesity are just a few conditions that increase the risk of fungal nail infection. Wearing artificial nails is associated with a greater risk of onychomycosis because the surface of the nail is often scraped and prepped, making the nail susceptible to infection.
Warm, humid conditions such as those found around pool areas, public locker rooms, and shower areas provide the perfect breeding ground for fungus to thrive.
An alternative name is dermatophytic onychomycosis.
Slideshow: See Pictures and Learn About Ringworm
Text Reference: American Academy of Dermatology: "Ringworm"