|Fungal infections can cause the nails to become thick and discolored. The discoloration is most often a shade of yellow. A toenail fungus infection can possibly cause the nail to break away from the nail bed, leaving a space between the nail plate and the nail bed. This allows for bacteria, germs, and fungus to live under the toenail and breed. Also, on some occasions there can be an odor coming from the toenail.