The living organism that causes fungal infections thrives in dark, damp environments.   Since toes are often covered by socks and shoes, they are the ideal location for fungal growth.  Fungus can cause the toenails to appear yellowish or brownish in color.  The nails may become thick over time and crumble.   The resulting appearance can be unsightly and the added thickness of the toenail can ultimately cause pain.   Fungal spores can be transferred onto bare feet in locations such as gyms and swimming pools.   Wearing some sort of sandals or water shoes can help keep feet from directly touching the floor.   If you are exposed to any threat of fungal infection, wash your feet and dry them thoroughly.
Feet are susceptible to nail problems of all kinds, including fungal nails, nail infections, and ingrown toenails.    It might be because of all the time they spend in dark, moist places, or it might be due to lack of attention they receive.   Whatever the cause, your podiatrist is well equipped to help provide the cure, from medication to surgery to a change in footwear.