Onychomycosis, or toenail fungus, is a contagious fungal infection. It often causes thickening of the nails, along with a yellow, white or brown discoloration. An extremely affected nail may also become frail and detach. Sometimes, toenail fungus can produce an unpleasant odour. Although numerous risk factors and causes have been established, toenail fungus is rarely harmful to your health. It is generally treated for its unaesthetic or painful attributes.