Fungus A type of microorganism that can grow on and infect nails, causing discoloration, thickening, and other symptoms. Related Articles: Nail Fungus in the SalonDifference Between Fungal Nail Infections and Nail PsoriasisOnychomadesis & Hand Foot Mouth DiseaseWhy Foot Hygiene in Hot Weather Matters More Than You ThinkMixing Nail Brands and Lamps