Onychomycosis, the Active Invasion of a Normal Nail Unit by a Dermatophytic Versus the Colonization of an Existing Abnormal Nail Unit by Environmental Fungus.

Onychomycosis was described by early investigators as the presence of an abnormal nail unit and a member of the order Mycota, producing the abnormality. This interpretation has caused more than 50 years of confusion in the dermatologic literature. Unquestionably, the clinician sees more abnormal toenails than fingernails, and investigators have described a multitude of fungi as the cause of the clinically abnormal toenail.