Treat onychomycosis, a fungal infection under the toenail, by applying over-the-counter remedies like Vicks VapoRub ($4), Lamisil ($11), or tea tree oil ($9). Onychomycosis is a fairly common infection that can cause discoloration in the nail and a thick skin to develop beneath it, especially at the corners. Look out for this if you go barefoot at swimming pools, gyms, or shower rooms.