A nail disease or onychosis is a disease or deformity of the nail. Although the nail is a structure produced by the skin and is a skin appendage, nail diseases have a distinct classification as they have their own signs and symptoms which may relate to other medical conditions. Some nail conditions that show signs of infection or inflammation may require medical assistance. Diseases
Onychia is an inflammation of the nail folds (surrounding tissue of the nail plate) of the nail with formation of pus and shedding of the nail. Onychia results from the introduction of microscopic pathogens through small wounds. Onychocryptosis, commonly known as "ingrown nails" (unguis incarnatus), can affect either the fingers or the toes. In this condition, the nail cuts into one or both sides of the nail bed, resulting in inflammation and possibly infection. The relative rarity of this condition in the fingers suggests that pressure from the ground or shoe against the toe is a prime factor. The movements involved in walking or other physical disturbances can contribute to the problem. Mild onychocryptosis, particularly in the absence of infection, can be treated by trimming and rounding the nail. More advanced cases, which usually include infection, are treated by surgically excising the ingrowing portion of the nail down to its bony origin and thermally or chemically cauterizing the matrix, or root, to prevent recurrence. This surgery is called matrixectomy. The best results are achieved by cauterizing the matrix with phenol.
The normal process of change is: brittle nails, straight nails, spoon-shaped nails. Subungual hematoma occurs when trauma to the nail results in a collection of blood, or hematoma, under the nail. It may result from an acute injury or from repeated minor trauma such as running in undersized shoes. Acute subungual hematomas are quite painful, and are usually treated by releasing the blood by creating a small hole in the nail. Drilling and thermal cautery  are common methods for creating the hole. Thermal cautery is not used on acrylic nails because they are flammable. Onychomatricoma, a tumor of the nail matrix. Nail pemphigus, an auto-immune disease. Erythronychia, red bands in the nail from some inflammatory conditions. Melanonychia, a black, brown or grey  discoloration of the nail, with numerous causes. Nail changes and conditions associated with them
Nail inspection can give hints to the internal condition of the body as well. Nail disease can be very subtle and should be evaluated by a dermatologist with a focus in this particular area of medicine. A nail technician may be the first to note a subtle change in nail health. Pliability
Brittleness is associated with iron deficiency, thyroid problems, and impaired kidney function. Splitting and fraying are associated with psoriasis and deficiencies of folic acid, protein, and Vitamin C.
Unusual thickness is associated with circulation problems. Shape and texture
Nail clubbing - nails that curve down around the fingertips with nailbeds that bulge is associated with oxygen deprivation and lung, heart, or liver disease.