They can determine the best course of action based on the severity of the infection and your individual health condition. Nail fungus, also known as onychomycosis, is a common fungal infection that affects the nails, usually of the toenails or fingernails. It is caused by various types of fungi, including dermatophytes, yeasts, and molds. Symptoms: Thickened nails Brittle or crumbly nails...