If you routinely find your toenails yellowed and deformed, fungi may be to blame. There is a type of fungus that thrives in dark, moist, and warm areas, much like your shoes, shower, locker room, etc. You may notice yellow and/or white spots on the nail, along with a thickness that develops over time. If one nail is infected and left untreated, it can easily spread to other nails.
Treatments include a laser cleaning of the nail(s), oral medications, and medicated or non medicated anti-fungal cream.