A fungus is an organism that lives in warm moist areas. Fungus of the toenails is a common problem that can affect people of all ages, although it most commonly affects individuals who are older.

Toenail fungus often begins as an infection in the skin called tinea pedis (also known as athlete’s foot). The fungus often starts under the nail fold at the end of the nail. Over time, it grows underneath the nail and causes changes to its appearance, such as a yellow or brownish discoloration. It can also cause thickening and deformity of the toenail.
Many people have difficulty with their toenails and need assistance in caring for them. A foot and ankle surgeon can diagnose the cause of toenail problems and can recommend treatments.

​Source:  https://www.foothealthfacts.org/conditions/nail-fungus