Fungal nail fungal infection is simply the infection of the nail by a yeast or mold. Fungal infection is commonly one of the commonest nail disorders among adults. It can also occur along the fingernails, especially where there are cuts and breaks in the skin. Fungal infection also known as onychomycosis is quite common in children and in very rare cases in adults; the condition occurs more often in women. The exact cause of fungal nail infection or onychomycosis is not known, although certain conditions may be present that enable the fungal growth, including prolonged contact with warm moist environments, poor nutrition and poor hygiene among others.