Onychomycosis (OM) or Fungal Nail infections are common. They're not serious but they can take a long time to treat and many patients dislike the appearance of them as they can cause the nail to become thickened and discoloured. Onychomycosis mainly occurs in your toenails but can present in your fingernails too. Fungal nail infections are caused by an overgrowth of fungi in, under, or on the nail. These fungi thrive in warm moist environments - (think sweaty socks or shoes.)