Onychomycosis of the nails includes fungal infection caused by primary nail pathogens that invade the healthy nail plate and secondary invasion in nails with preexisting disease. Onychomycosis may be due to secondary infection from tinea pedis and is the most common nail disease, accounting for roughly 30% of all cutaneous fungal infections. In a survey of 9332 British adults, 16 years of age and older, the prevalence of dermatophyte infection was 2.6% to 2.8%. In subjects 55 years of age or older, the prevalence was 4.7%. In the United States, a similar prevalence has been reported. Onychomycosis can have a significant psychological effect on the subject. In a North American study, the relative composition of isolates from nails was dermatophytes (91%), yeasts (5%), and molds (4%). Candida is isolated predominantly from fingernails. Uncommonly, there are mixed infections. In other parts of the world, the frequency of these agents may vary.