Onychomycosis is a fungal infection of the fingernails and/or toenails. It is caused by a fungus which grows within the nail and causes the nail bed to separate from the nail. The nail may appear to crumble or become raised up and have a yellowish-brown discoloration. Systemic treatment is often required to eradicate the fungus. Topical therapies are also used, but they are less effective.