Onychomycosis is usually asymptomatic. Sometimes, however, it is painful, especially when there is a secondary bacterial infection or when the dystrophic growth pattern causes an ingrown toenail. In a small number of extremely severe cases, there can be difficulty in ambulation. Affected nails become yellow, thickened, and onycholytic, meaning that the nail plate partially separates from the nail bed (Fig. 71-1). Toenails are more often affected than fingernails; one common pattern is the infection of both feet, but only one hand. It is common for only some nails on a foot or hand to be involved, whereas neighboring nails are spared.

Onychomycosis is extremely common. Total prevalence in the United States is in the range of 5% to 10%, and up to 20% of individuals over the age of 40 will have this problem at some point in their lives. The most common agent is Trichophyton rubrum.