The main cause of toenail fungus is a type of mold known as dermatophytes. Dermatophytes are fungal microorganisms that feed off keratin, the protein found in your fingernails and toenails that makes them hard. These dermatophytes are responsible for 90% of toenail fungal infections, with other less common types of fungi making up the last 10%. It’s important to note that toenail fungus is quite contagious, often spreading through direct contact with someone or by touching an infected surface. This is something to be mindful of if you have a fungal infection or think that you may have one.