The fungus that commonly grows under toenails is called onychomycosis and tinea unguium, according to Mayo Clinic. The fungus appears as white or yellow spots under fingernails or toenails.
Nail fungus can lead to discoloration, thickening or crumbling of the nail. The infected nail may also become distorted or lose its shine, according to Mayo Clinic. The fungus can spread to multiple nails, but usually does not infect all of them. More severe cases can be painful and require medical treatment. Keeping feet dry from water and sweat can help prevent fungal infections, because fungi thrive in warm, damp, dark places.