Fungal nails (onychomycosis) may be caused by many species of fungi, but the most common is Trichophyton rubrum. Distal subungal onychomycosis starts as a discolored area at the nail's corner and slowly spread toward the cuticle. In proximal subungal onychomycosis, the infection starts at the cuticle and spreads toward the nail tip. Yeast onychomycosis is caused by Candida and may be the most common cause of fungal fingernail.
The causative pathogens of onychomycosis are all in the fungus kingdom and include dermatophytes, Candida (yeasts), and nondermatophytic molds. Dermatophytes are the fungi most commonly responsible for onychomycosis in the temperate western countries; while Candida and nondermatophytic molds are more frequently involved in the tropics and subtropics with a hot and humid climate.
In normal, healthy people, fungal infections of the nails are most commonly caused by fungus that is caught from moist, wet areas. Communal showers, such as those at a gym or swimming pools, are common sources. Going to nail salons that use inadequate sanitization of instruments (such as clippers, filers, and foot tubs) in addition to living with family members who have fungal nails are also risk factors. Athletes have been proven to be more susceptible to nail fungus. This is presumed to be due to the wearing of tight-fitting, sweaty shoes associated with repetitive trauma to the toenails. Having athlete's foot makes it more likely that the fungus will infect your toenails. Repetitive trauma also weakens the nail, which makes the nail more susceptible to fungal infection.
Use soap and water to wash your feet, and dry well, including between toes. Trim your toenails -- straight across -- to keep them shorter than the end of your toe. Make sure the tools you use are clean, too. Wash clippers and files with soap and water, then wipe with rubbing alcohol. You might be tempted to cover up discolored nails with polish, but don't. Your nail bed can't "breathe," which keeps fungus from going away.