The skin typically presents as red and hot. These infections can be painful.

Pus is usually present, along with gradual thickening and browning discoloration of the nail plate.

Paronychia may be divided as follows:

- "Acute paronychia" is an infection of the folds of tissue surrounding the nail of a finger or, less commonly, a toe, lasting less than six weeks. The infection generally starts in the paronychium at the side of the nail, with local redness, swelling, and pain. Acute paronychia is usually caused by direct or indirect trauma to the cuticle or nail fold, and may be from relatively minor events, such as dishwashing, an injury from a splinter or thorn, nail biting, biting or picking at a hangnail, finger sucking, an ingrown nail, or manicure procedures.

- "Chronic paronychia" is an infection of the folds of tissue surrounding the nail of a finger or, less commonly, a toe, lasting more than six weeks. It is a nail disease prevalent in individuals whose hands or feet are subject to moist local environments, and is often due to contact dermatitis. In chronic paronychia, the cuticle separates from the nail plate, leaving the region between the proximal nail fold and the nail plate vulnerable to infection. It can be the result of dish washing, finger sucking, aggressively trimming the cuticles, or frequent contact with chemicals (mild alkalis, acids, etc.).

Alternatively, paronychia may be divided as follows:

- "Candidal paronychia" is an inflammation of the nail fold produced by "Candida albicans".

- "Pyogenic paronychia" is an inflammation of the folds of skin surrounding the nail caused by bacteria. Generally acute paronychia is a pyogenic paronychia as it is usually caused by a bacterial infection.

The most common symptom of a fungal nail infection is the nail becoming thickened and discoloured: white, black, yellow or green. As the infection progresses the nail can become brittle, with pieces breaking off or coming away from the toe or finger completely. If left untreated, the skin underneath and around the nail can become inflamed and painful. There may also be white or yellow patches on the nailbed or scaly skin next to the nail, and a foul smell. There is usually no pain or other bodily symptoms, unless the disease is severe. People with onychomycosis may experience significant psychosocial problems due to the appearance of the nail, particularly when fingers – which are always visible – rather than toenails are affected.

Dermatophytids are fungus-free skin lesions that sometimes form as a result of a fungus infection in another part of the body. This could take the form of a rash or itch in an area of the body that is not infected with the fungus. Dermatophytids can be thought of as an allergic reaction to the fungus.

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.

Onychomycosis, also known as tinea unguium, is a fungal infection of the nail. This condition may affect toenails or fingernails, but toenail infections are particularly common.

Treatment may be based on the signs. Treatment may be with the medication terbinafine.

It occurs in about 10 percent of the adult population. It is the most common disease of the nails and constitutes about half of all nail abnormalities.

The term is from Ancient Greek ὄνυξ "ónux" "nail", μύκης "múkēs" "fungus" and -ωσις "ōsis" "functional disease."

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.

Candidal onychomycosis is an infection of the nail plate by fungus caused by "Candida". In one study "Candida parapsilosis" was the most common species; "Candida albicans" is also a common agent.

A hangnail is a small, torn piece of skin, more specifically eponychium or paronychium, next to a fingernail or toenail.

Periungual warts are warts that cluster around the fingernail or toenail. They appear as thickened, fissured cauliflower-like skin around the nail plate. Periungual warts often cause loss of the cuticle and paronychia. Nail biting increases susceptibility to these warts.

Warts of this kind often cause damage to the nail either by lifting the nail from the skin or causing the nail to partially detach. If they extend under the nail, then the patient may suffer pain as a result. Sometimes periungual wart infections resemble the changes that are found in onychomycosis. In worst cases, if the infection causes injury or damage to the nail matrix, deformity in the nail may become permanent.

As with other wart types, a number of treatments are available, including laser therapy, cryotherapy, salicylic acid, and other topical treatments.

Mycoses are classified according to the tissue levels initially colonized.

Superficial mycoses are limited to the outermost layers of the skin and hair.

An example of such a fungal infection is "Tinea versicolor", a fungus infection that commonly affects the skin of young people, especially the chest, back, and upper arms and legs. "Tinea versicolor" is caused by a fungus that lives in the skin of some adults. It does not usually affect the face. This fungus produces spots that are either lighter than the skin or a reddish brown. This fungus exists in two forms, one of them causing visible spots. Factors that can cause the fungus to become more visible include high humidity, as well as immune or hormone abnormalities. However, almost all people with this very common condition are healthy.

Oral lichen planus (also termed "oral mucosal lichen planus"), is a form of mucosal lichen planus, where lichen planus involves the oral mucosa, the lining of the mouth. This may occur in combination with other variants of lichen planus. Six clinical forms of oral lichen planus are recognized:

- "Reticular", the most common presentation of oral lichen planus, is characterised by the net-like or spider web-like appearance of lacy white lines, oral variants of Wickham's straiae. This is usually asymptomatic.

- "Erosive/ulcerative", the second most common form of oral lichen planus, is characterised by oral ulcers presenting with persistent, irregular areas of redness, ulcerations and erosions covered with a yellow slough. This can occur in one or more areas of the mouth. In 25% of people with erosive oral lichen planus, the gums are involved, described as desquamative gingivitis (a condition not unique to lichen planus). This may be the initial or only sign of the condition.

- "Papular," with white papules.

- "Plaque-like" appearing as a white patch which may resemble leukoplakia.

- "Atrophic," appearing as areas. Atrophic oral lichen planus may also manifest as desquamative gingivitis.

- "Bullous," appearing as fluid-filled vesicles which project from the surface.

These types often coexist in the same individual. Oral lichen planus tends to present bilaterally as mostly white lesions on the inner cheek, although any mucosal site in the mouth may be involved. Other sites, in decreasing order of frequency, may include the tongue, lips, gingivae, floor of the mouth, and very rarely, the palate.

Generally, oral lichen planus tends not to cause any discomfort or pain, although some people may experience soreness when eating or drinking acidic or spicy foodstuffs or beverages. When symptoms arise, they are most commonly associated with the atrophic and ulcerative subtypes. These symptoms can include a burning sensation to severe pain. Lichen planus, particularly when concomitant oral or genital lesions occur, significantly affects patients’ quality of life.

Lichen planus affecting mucosal surfaces may have one lesion or be multifocal. Examples of lichen planus affecting mucosal surfaces include:

- "Esophageal lichen planus", affecting the esophageal mucosa. This can present with difficulty or pain when swallowing due to oesophageal inflammation, or as the development of an esophageal stricture. It has also been hypothesized that it is a precursor to squamous cell carcinoma of the esophagus.

- "Genital lichen planus," which may cause lesions on the glans penis or skin of the scrotum in males, and the vulva or vagina in females. Symptoms may include lower urinary tract symptoms associated with stenosis of the urethra, painful sexual intercourse, and itching. In females, "Vulvovaginal-gingival syndrome," is severe and distinct variant affecting the vulva, vagina, and gums, with complications including scarring, vaginal stricture formation, or vulva destruction. The corresponding syndrome in males, affecting the glans penis and gums, is the "peno-gingival syndrome". It is associated with HLA-DQB1.

Hangnails can become infected and cause paronychia, a type of skin infection that occurs around the nails. Treatments for paronychia vary with severity, but may include soaking in hot salty water, the use of oral antibiotic medication, or clinical lancing. Paronychia itself rarely results in further complications but can lead to abscess, permanent changes to the shape of the nail or the spread of infection.

"Onycholysis" is from "onycho-", from Ancient Greek ὄνυξ "ónuks", meaning "nail", and Ancient Greek λύσις "lúsis", meaning "a loosening".

Symptoms of an ingrown nail include pain along the margins of the nail (caused by hypergranulation that occurs around the aforementioned margins), worsening of pain when wearing tight footwear, and sensitivity to pressure of any kind, even the weight of bedsheets. Bumping of an affected toe can produce sharp and even excruciating pain as the tissue is punctured further by the nail. By the very nature of the condition, ingrown nails become easily infected unless special care is taken early to treat the condition by keeping the area clean. Signs of infection include redness and swelling of the area around the nail, drainage of pus and watery discharge tinged with blood. The main symptom is swelling at the base of the nail on the ingrowing side (though it may be both sides).

Onychocryptosis should not be confused with a similar nail disorder, convex nail, nor with other painful conditions such as involuted nails, nor with the presence of small corns, callus or debris down the nail sulci (grooves on either side).

Longitudinal erythronychia presents with longitudinal red bands in the nail plate that commence in the matrix and extend to the point of separation of the nail plate and nailbed, and may occur on multiple nails with inflammatory conditions such as lichen planus or Darier's disease.

Onycholysis is a common medical condition characterized by the painless detachment of the nail from the nail bed, usually starting at the tip and/or sides. On the hands, it occurs particularly on the ring finger but can occur on any of the fingernails. It may also happen to toenails.

Onycholysis can occur in many conditions, including psoriasis. In thyrotoxicosis it is thought to be due to sympathetic overactivity. It may also be seen in infections or trauma.

Green nail syndrome, also known as chloronychia, is a paronychial infection caused due to "Pseudomonas aeruginosa" that can develop in individuals whose hands are frequently submerged in water. It may also occur as transverse green stripes that are ascribed to intermittent episodes of infection.

Many conditions affect the human integumentary system—the organ system covering the entire surface of the body and composed of skin, hair, nails, and related muscle and glands. The major function of this system is as a barrier against the external environment. The skin weighs an average of four kilograms, covers an area of two square meters, and is made of three distinct layers: the epidermis, dermis, and subcutaneous tissue. The two main types of human skin are: glabrous skin, the hairless skin on the palms and soles (also referred to as the "palmoplantar" surfaces), and hair-bearing skin. Within the latter type, the hairs occur in structures called pilosebaceous units, each with hair follicle, sebaceous gland, and associated arrector pili muscle. In the embryo, the epidermis, hair, and glands form from the ectoderm, which is chemically influenced by the underlying mesoderm that forms the dermis and subcutaneous tissues.

The epidermis is the most superficial layer of skin, a squamous epithelium with several strata: the stratum corneum, stratum lucidum, stratum granulosum, stratum spinosum, and stratum basale. Nourishment is provided to these layers by diffusion from the dermis, since the epidermis is without direct blood supply. The epidermis contains four cell types: keratinocytes, melanocytes, Langerhans cells, and Merkel cells. Of these, keratinocytes are the major component, constituting roughly 95 percent of the epidermis. This stratified squamous epithelium is maintained by cell division within the stratum basale, in which differentiating cells slowly displace outwards through the stratum spinosum to the stratum corneum, where cells are continually shed from the surface. In normal skin, the rate of production equals the rate of loss; about two weeks are needed for a cell to migrate from the basal cell layer to the top of the granular cell layer, and an additional two weeks to cross the stratum corneum.

The dermis is the layer of skin between the epidermis and subcutaneous tissue, and comprises two sections, the papillary dermis and the reticular dermis. The superficial papillary dermis with the overlying rete ridges of the epidermis, between which the two layers interact through the basement membrane zone. Structural components of the dermis are collagen, elastic fibers, and ground substance. Within these components are the pilosebaceous units, arrector pili muscles, and the eccrine and apocrine glands. The dermis contains two vascular networks that run parallel to the skin surface—one superficial and one deep plexus—which are connected by vertical communicating vessels. The function of blood vessels within the dermis is fourfold: to supply nutrition, to regulate temperature, to modulate inflammation, and to participate in wound healing.

The subcutaneous tissue is a layer of fat between the dermis and underlying fascia. This tissue may be further divided into two components, the actual fatty layer, or panniculus adiposus, and a deeper vestigial layer of muscle, the panniculus carnosus. The main cellular component of this tissue is the adipocyte, or fat cell. The structure of this tissue is composed of septal (i.e. linear strands) and lobular compartments, which differ in microscopic appearance. Functionally, the subcutaneous fat insulates the body, absorbs trauma, and serves as a reserve energy source.

Conditions of the human integumentary system constitute a broad spectrum of diseases, also known as dermatoses, as well as many nonpathologic states (like, in certain circumstances, melanonychia and racquet nails). While only a small number of skin diseases account for most visits to the physician, thousands of skin conditions have been described. Classification of these conditions often presents many nosological challenges, since underlying etiologies and pathogenetics are often not known. Therefore, most current textbooks present a classification based on location (for example, conditions of the mucous membrane), morphology (chronic blistering conditions), etiology (skin conditions resulting from physical factors), and so on. Clinically, the diagnosis of any particular skin condition is made by gathering pertinent information regarding the presenting skin lesion(s), including the location (such as arms, head, legs), symptoms (pruritus, pain), duration (acute or chronic), arrangement (solitary, generalized, annular, linear), morphology (macules, papules, vesicles), and color (red, blue, brown, black, white, yellow). Diagnosis of many conditions often also requires a skin biopsy which yields histologic information that can be correlated with the clinical presentation and any laboratory data.

Melanonychia is a black or brown pigmentation of the normal nail plate, and may be present as a normal finding on many digits in African-Americans, as a result of trauma, systemic disease, or medications, or as a postinflammatory event from such localized events as lichen planus or fixed drug eruption.

There are two types, longitudinal and transverse melanonychia. Longitudinal melanonychia may be a sign of subungual melanoma (acral lentiginous melanoma), although there are other diagnoses such as chronic paronychia, onychomycosis, subungual hematoma, pyogenic granuloma, glomus tumour, subungual verruca, mucous cyst, subungual fibroma, keratoacanthoma, carcinoma of the nail bed, and subungual exostosis.

Psoriatic nails are characterized by a translucent discolouration in the nail bed that resembles a drop of oil beneath the nail plate. Early signs that may accompany the "oil drop" include thickening of the lateral edges of the nail bed with or without resultant flattening or concavity of the nail; separation of the nail from the underlying nail bed, often in thin streaks from the tip-edge to the cuticle; sharp peaked "roof-ridge" raised lines from cuticle to tip; or separation of superficial layers of the nail followed by loss of patches of these superficial layers, leaving thin red nails beneath; or nail pitting–punctate changes along the nail plate surface.

Onychomatricoma is a cutaneous condition characterized by a distinctive tumor of the nail matrix.

This nail disease can mimic many nail problems and should be examined and biopsied by a dermatologist. In particular, a main concern is the malignant and destructive potential that may exist.

Longitudinal leukonychia is far less common and features smaller 1mm white longitudinal lines visible under the nail plate. It may be associated with Darier's disease.

An ingrown nail (also known as onychocryptosis from ὄνυξ ("onyx", "nail") + κρυπτός ("kryptos", "hidden") or unguis incarnates) is a common form of nail disease. It is an often painful condition in which the nail grows so that it cuts into one or both sides of the paronychium or nail bed.

The common opinion is that the nail enters inside the paronychium, but an ingrown toenail can simply be overgrown toe skin. The condition starts from a microbial inflammation of the paronychium, then a granuloma, which results in a nail buried inside of the granuloma. While ingrown nails can occur in the nails of both the hands and the feet , they occur most commonly with the toenails.

A true ingrown toenail is caused by the actual penetration of flesh by a sliver of nail.