Onychomycosis, or nail infection, is a persistent fungal infection of the nail bed, matrix or plate. It is the most common nail disorder in adults, accounting for one-third of all fungal skin infections and up to 50 percent of all nail diseases. Toenails are affected more often than fingernails. The infection can cause nails to become yellow or discoloured. Advanced infection causes thick, brittle nails that often separate from the nail bed. It can also cause discomfort when wearing shoes or walking.