Toenail fungus is an infection of the nail bed on one or more toes. Fungus can easily spread from one nail to another, or from one person to another, if in close enough contact.
Dermophytes, a category of fungi, attack the nail and eat away at the keratin (protein) within the nail. This causes the classic fungal symptoms of growing thick, brittle, discolored nails. In extreme circumstances, the nails can have a foul odor, and even lift off the nail bed.