Common causes include localized infection, minor injury to the matrix bed, or severe systemic illness. It is sometimes a side effect of chemotherapy or x-ray treatments for cancer. A new nail plate will form once the cause of the disease is removed. Onychomycosis, also known as tinea unguium, is a contagious infection of the nail caused by the same fungal organisms which cause ringworm of the skin (Trichophyton rubrum or T. mentagrophytes, rarely other trichophyton species or Epidermophyton floccosum ). It can result in discoloration, thickening, chalkiness, or crumbling of the nails and is often treated by powerful oral medications which, rarely, can cause severe side effects including liver failure. Mild onychomycosis sometimes responds to a combination of topical antifungal medication, sometimes applied as special medicinal nail lacquer, and periodic filing of the nail surface. For advanced onychomycosis, especially if more than one nail is infected, systemic medication (pills) is preferred. Home remedies are often used, although their effectiveness is disputed. Onychophosis is a growth of horny epithelium in the nail. Onychoptosis is the periodic shedding of one or more nails, in whole or part. This condition may follow certain diseases such as syphilis, or can result from fever, trauma, systemic upsets or adverse reaction to drugs. Onychorrhexis also known as brittle nails, is brittleness with breakage of fingernails or toenails. Paronychia is a bacterial or fungal infection where the nail and skin meet. Koilonychia is when the nail curves upwards (becomes spoon-shaped) due to an iron deficiency.
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.