Fungus. A fungus may colonize a nail. This is common on toes and sometimes fingers. It is much more common among people who sleep with sox on their feet. Chronic- recurrent foot fungus is seen with it. Some people seem to be more likely to have it as a family trait, probably a genetic variation. Sometimes it is seen on 2 feet but just 1 hand. It can be treated. Dermatophytes are never systemic!
Toenail fungus. Toenail fungus is a manifestation of a systemic issue. These organisms will colonize in the gut. When the fungal infection enters the bloodstream, the immune system will attempt to discharge it through skin and skin structures. It is not dangerous, but can be highly resistant to topical therapy since it is continually discharged into the nail bed. You must treat the causes which are diet related.