A nail fungus infection as implied by its name is caused by a fungus. The infection can affect both the fingernails and toenails. It more commonly appears in toenails than fingernails. This type of infection is also medically known as onychomycosis.
Nail fungus can cause changes in the appearance of the infected nails in many ways. These include nail discoloration (often white or yellow), the nail turning brittle, crumbly, and having jagged edges. The nail may also thicken and separate from the nail bed. At times, it may completely fall off. The nail may also curl up or down. In some cases, it can be painful and it may smell bad too.