Nail diseases are called onychosis. They may be a part of general diseases like psoriasis, lupus, anemia and many more, whose symptoms manifest on nails too. They can also appear as independent diseases, most frequently those caused by fungi. The presence of the fungi is determined by microscopic examination of the material taken from the nail surface.
Diseases related to peritoneal tissue, and that is the tissue that surrounds nail, also belong to this group of diseases.