Onychomycosis, also known as tinea unguium, is a fungal infection of the nail. Symptoms may include white or yellow nail discoloration, thickening of the nail, and separation of the nail from the nail bed. Toenails or fingernails maybe affected, but it is more common for toenails to be affected. A number of different types of fungus can cause onychomycosis including dermatophytes and Fusarium. Risk factors include athlete's foot, other nail diseases, exposure to someone with the condition, peripheral vascular disease, and poor immune function. The diagnosis is generally suspected based on the appearance and confirmed by laboratory testing. It occurs in about 10 percent of the adult population. Older people are more frequently affected. Males are affected more often than females. Onychomycosis represents about half of nail disease.
As the infection progresses the nail can become brittle, with pieces breaking off or coming away from the toe or finger completely. If left untreated, the skin underneath and around the nail can become inflamed and painful. There may also be white or yellow patches on the nailbed or scaly skin next to the nail, and a foul smell. There is usually no pain or other bodily symptoms, unless the disease is severe. People with onychomycosis may experience significant psychosocial problems due to the appearance of the nail, particularly when fingers – which are always visible – rather than toenails are affected.
The causative pathogens of onychomycosis are all in the fungus kingdom and include dermatophytes, Candida, and nondermatophytic molds. 
Trichophyton rubrum is the most common dermatophyte involved in onychomycosis. Other dermatophytes that may be involved are T. interdigitale, Epidermophyton floccosum, T. violaceum, Microsporum gypseum, T. tonsurans, and T. soudanense. 
Other causative pathogens include Candida and nondermatophytic molds, in particular members of the mold genus Scytalidium, Scopulariopsis, and Aspergillus. Candida species mainly cause fingernail onychomycosis in people whose hands are often submerged in water. Scytalidium mainly affects people in the tropics, though it persists if they later move to areas of temperate climate.
Other molds more commonly affect people older than 60 years, and their presence in the nail reflects a slight weakening in the nail's ability to defend itself against fungal invasion.
The diagnosis is generally suspected based on the appearance and confirmed by laboratory testing. The four main tests are a KOH smear, culture, histology examination, and polymerase chain reaction. The sample examined is generally nail scrapings or clippings. 
Nail plate biopsy with periodic acid-Schiff stain appear more useful than culture or direct KOH examination. To reliably identify nondermatophyte molds, several samples may be necessary.
 
There are four classic types of onychomycosis: