Common organisms include Group A "Streptococcus" (group A strep), "Klebsiella", "Clostridium", "Escherichia coli", "Staphylococcus aureus," and "Aeromonas hydrophila", and others. Group A strep is considered the most common cause of necrotizing fasciitis.

The majority of infections are caused by organisms that normally reside on the individual's skin. These skin flora exist as commensals and infections reflect their anatomical distribution (e.g. perineal infections being caused by anaerobes).

Sources of MRSA may include working at municipal waste water treatment plants, exposure to secondary waste water spray irrigation, exposure to run off from farm fields fertilized by human sewage sludge or septage, hospital settings, or sharing/using dirty needles. The risk of infection during regional anesthesia is considered to be very low, though reported.

Vibrio vulnificus, a bacterium found in saltwater, is a rare cause.

More than 70% of cases are recorded in people with at least one of the following clinical situations: immunosuppression, diabetes, alcoholism/drug abuse/smoking, malignancies, and chronic systemic diseases. For reasons that are unclear, it occasionally occurs in people with an apparently normal general condition.

The infection begins locally at a site of trauma, which may be severe (such as the result of surgery), minor, or even non-apparent.

Cellulitis in 2015 resulted in about 16,900 deaths worldwide, up from 12,600 in 2005.

The elderly and those with a weakened immune system are especially vulnerable to contracting cellulitis. Diabetics are more susceptible to cellulitis than the general population because of impairment of the immune system; they are especially prone to cellulitis in the feet, because the disease causes impairment of blood circulation in the legs, leading to diabetic foot or foot ulcers. Poor control of blood glucose levels allows bacteria to grow more rapidly in the affected tissue, and facilitates rapid progression if the infection enters the bloodstream. Neural degeneration in diabetes means these ulcers may not be painful, thus often become infected. Those who have suffered poliomyelitis are also prone because of circulatory problems, especially in the legs.

Immunosuppressive drugs, and other illnesses or infections that weaken the immune system, are also factors that make infection more likely. Chickenpox and shingles often result in blisters that break open, providing a gap in the skin through which bacteria can enter. Lymphedema, which causes swelling on the arms and/or legs, can also put an individual at risk.

Diseases that affect blood circulation in the legs and feet, such as chronic venous insufficiency and varicose veins, are also risk factors for cellulitis.

Cellulitis is also common among dense populations sharing hygiene facilities and common living quarters, such as military installations, college dormitories, nursing homes, oil platforms, and homeless shelters.

The most common digit to become ingrown is the big toe, but ingrowth can occur on any nail. Ingrown nails can be avoided by cutting nails straight across; not along a curve, not too short and no shorter than the flesh around it. Footwear that is too small or too narrow, or with too shallow of a "toe box", will exacerbate any underlying problem with a toenail. Sharp square corners may be uncomfortable and cause snagging on socks. Proper cutting leaves the leading edge of the nail free of the flesh, precluding it from growing into the toe. Filing of the corner is reasonable. Some nails require cutting of the corners far back to remove edges that dig into the flesh; this is often done as a partial wedge resection by a podiatrist. Ingrown toe nails can be caused by injury, commonly blunt trauma in which the flesh is pressed against the nail causing a small cut that swells. Injury to the nail can cause it to grow abnormally, making it wider or thicker than normal, or even bulged or crooked.

The main contributor to onychocryptosis is footwear, particularly ill-fitting shoes with inadequate toe-box room and tight stockings that apply pressure to the top or side of the foot. Other factors may include the damp atmosphere of enclosed shoes, which soften the nail-plate and cause swelling on the epidermal keratin (eventually increasing the convex arch permanently), genetics, trauma and disease. Improper cutting of the nail may cause the nail to cut into the side-fold skin from growth and impact, whether or not the nail is truly "ingrown". The nail bends inwards or upwards depending on the angle of its cut. If the cutting tool, such as scissors, is at an attitude in which the lower blade is closer to the toe than the upper blade, the toenail will tend to grow upwards from its base, and vice versa. The process is visible along the nail as it grows, appearing as a warp advancing to the end of the nail. The upper corners turn more easily than the center of the nail tip. Holding the tool at the same angle for all nails may induce these conditions; as the nail turns closer to the skin, it becomes harder to fit the lower blade in the right attitude under the nail. When cutting a nail, it is not just the correct angle that is important, but also how short it is cut. A shorter cut will bend the nail more, unless the cut is even on both top and bottom of the nail.

Causes may include:

- Shoes causing a bunching of the toes in the developmental stages of the foot (frequently in people under 21), which can cause the nail to curl and dig into the skin. This is particularly the case in ill-fitting shoes that are too narrow or too short, but any toed shoes may cause an ingrown nail.

- Poor nail care, including cutting the nail too short, rounded off at the tip or peeled off at the edges instead of being cut straight across.

- Broken toenails.

- Trauma to the nail plate or toe, which can occur by dropping objects on or stubbing the toenail, or by the nail protruding through the shoe (as during sports or other vigorous activity), can cause the flesh to become injured and the nail to grow irregularly and press into the flesh.

- Predisposition, such as abnormally shaped nail beds, nail deformities caused by diseases or a genetic susceptibility, increases the chance of an ingrown nail, but the ingrowth cannot occur without pressure from a shoe.

- A bacterial infection, treatable with antibiotics.

One study compared patients with ingrown toenails to healthy controls and found no difference in the shape of toenails between those of patients and of the control group. The study suggested that treatment should not be based on the correction of a non-existent nail deformity. Ingrown toenails are caused by weight-bearing (activities such as walking, running, etc.) in patients that have too much soft skin tissue on the sides of their nail. Weight bearing causes this excessive amount of skin to bulge up along the sides of the nail. The pressure on the skin around the nail results in the tissue being damaged, resulting in swelling, redness and infection. Many treatments are directed at the nail itself and often include partial or full removal of the healthy nail. However, failure to treat the cutaneous condition can result in a return of the ingrowth and a deformity or mutilation of the nail.

This disease is most common among the elderly, infants, and children. People with immune deficiency, diabetes, alcoholism, skin ulceration, fungal infections, and impaired lymphatic drainage (e.g., after mastectomy, pelvic surgery, bypass grafting) are also at increased risk.

Most cases of erysipelas are due to "Streptococcus pyogenes" (also known as beta-hemolytic group A streptococci), although non-group A streptococci can also be the causative agent. Beta-hemolytic, non-group A streptococci include "Streptococcus agalactiae", also known as group B strep or GBS. Historically, the face was most affected; today, the legs are affected most often. The rash is due to an exotoxin, not the "Streptococcus" bacteria, and is found in areas where no symptoms are present; e.g., the infection may be in the nasopharynx, but the rash is found usually on the upper dermis and superficial lymphatics.

Erysipelas infections can enter the skin through minor trauma, insect bites, dog bites, eczema, athlete's foot, surgical incisions and ulcers and often originate from streptococci bacteria in the subject's own nasal passages. Infection sets in after a small scratch or abrasion spreads, resulting in toxaemia.

Erysipelas does not affect subcutaneous tissue. It does not release pus, only serum or serous fluid. Subcutaneous edema may lead the physician to misdiagnose it as cellulitis, but the style of the rash is much more well circumscribed and sharply marginated than the rash of cellulitis.

Beau's lines are deep grooved lines that run from side to side on the fingernail or the toenail. They may look like indentations or ridges in the nail plate. This condition of the nail was named by a French physician, Joseph Honoré Simon Beau (1806–1865), who first described it in 1846.

Beau's lines are horizontal, going across the nail, and should not be confused with vertical ridges going from the bottom (cuticle) of the nail out to the fingertip. These vertical lines are usually a natural consequence of aging and are harmless. Beau's lines should also be distinguished from Muehrcke's lines of the fingernails. While Beau's lines are actual ridges and indentations in the nail plate, Muehrcke lines are areas of hypopigmentation without palpable ridges; they affect the underlying nail bed, and not the nail itself. Beau's lines should also be distinguished from Mees' lines of the fingernails, which are areas of discoloration in the nail plate.

There are several causes of Beau's lines. It is believed that there is a temporary cessation of cell division in the nail matrix. This may be caused by an infection or problem in the nail fold, where the nail begins to form, or it may be caused by an injury to that area. Some other reasons for these lines include trauma, coronary occlusion, hypocalcaemia, and skin disease. They may be a sign of systemic disease, or may also be caused by an illness of the body, as well as drugs used in chemotherapy or malnutrition. Beau's lines can also be seen one to two months after the onset of fever in children with Kawasaki disease.

Human nails grow at a rate which varies with many factors: age, and the finger or toe in question as well as nutrition. However, typically in healthy populations fingernails grow at about 0.1mm/day and toenails at about 0.05mm/day. With this in mind the date of the stress causing Beau's lines and other identifiable marks on nails can be estimated. As the nail grows out, the ridge visibly moves upwards toward the nail edge. When the ridge reaches the nail edge, the fingertips can become quite sore due to the mis-shapen nail pressing into the flesh deeper than usual, exposing the sensitive nail bed (the quick) at the nail edge.

A researcher found Beau's lines in the fingernails of two of six divers following a deep saturation dive to a pressure equal to 305 meters of sea water, and in six of six divers following a similar dive to 335 meters. They have also been seen in Ötzi the Iceman.

Degenerative disease is the result of a continuous process based on degenerative cell changes, affecting tissues or organs, which will increasingly deteriorate over time, whether due to normal bodily wear or lifestyle choices such as exercise or eating habits. Degenerative diseases are often contrasted with infectious diseases.

Unlike frostbite, trench foot does not require freezing temperatures; it can occur in temperatures up to 16° Celsius (about 60° Fahrenheit) and within as little as 13 hours. Exposure to these environmental conditions causes deterioration and destruction of the capillaries and leads to morbidity of the surrounding flesh. Excessive sweating (hyperhidrosis) has long been regarded as a contributory cause; unsanitary, cold, and wet conditions can also cause trench foot.

Trench foot can be prevented by keeping the feet clean, warm, and dry. It was also discovered in World War I that a key preventive measure was regular foot inspections; soldiers would be paired and each made responsible for the feet of the other, and they would generally apply whale oil to prevent trench foot. If left to their own devices, soldiers might neglect to take off their own boots and socks to dry their feet each day, but if it were the responsibility of another, this became less likely. Later on in the war, instances of trench foot began to decrease, probably as a result of the introduction of the aforementioned measures; of wooden duckboards to cover the muddy, wet, cold ground of the trenches; and of the increased practice of troop rotation, which kept soldiers from prolonged time at the front.

Lichen nitidus is a chronic inflammatory disease of unknown cause characterized by 1–2 mm, discrete and uniform, shiny, flat-topped, pale flesh-colored or reddish-brown papules that may appear as hypopigmented against dark skin. Occasionally, minimal scaling is present or can be induced by rubbing the surface of the papules. The disease usually affects children and young adults and is painless and usually nonpruritic, although protracted itching may occur in some cases. It is sometimes referred to by dermatologists as "mini lichen planus".

Linear arrangements of these papules is common (referred to as a Koebner phenomenon), especially on the forearms, but may occasionally be grouped, though not confluent, on flexural areas. Generally, the initial lesions are localized, and remain so, to the chest, abdomen, glans penis, and flexor aspects of the upper extremities; however, less commonly, the disease process can (1) be strictly isolated to the palms and soles, presenting with many hyperkeratotic, yellow papules that may coalesce into plaques that fissure or “...sometimes a non-specific keratoderma resembling chronic eczema,” or (2) become more widespread, with papules widely distributed on the body—the extensor surfaces of the elbows, wrists, and hands, folds of the neck, submammary region in females, groin, thighs, ankles, and feet—and fusing into erythematous, minimally scaled plaques, with reddness that develops tints of violet, brown, and yellow.

Pica is a craving for nonedible items such as dirt or clay. It is caused by iron deficiency which is normal during pregnancy and can be overcome with iron in prenatal vitamins or, if severe, parenteral iron

Haemorrhoids (piles) are swollen veins at or inside the anal area, resulting from impaired venous return, straining associated with constipation, or increased intra-abdominal pressure in later pregnancy. They are more common in pregnant than non-pregnant women. It is reported by 16% of women at 6 months postpartum. Most pregnant women in countries where the diet is not heavily fiber-based may develop hemorrhoids, although they will usually be asymptomatic. Hemorrhoids can cause bleeding, itching, soiling or pain, and they can become strangulated. Symptoms may resolve spontaneously after pregnancy, although hemorrhoids are also common in the days after childbirth. Conservative treatments for hemorrhoids in pregnancy include dietary modification, local treatments, bowel stimulants or depressants, or phlebotonics (to strengthen capillaries and improve microcirculation). Treatment with oral hydroxyethylrutosides may help improve first and second degree hemorrhoids, but more information on safety in pregnancy is needed. Other treatments and approaches have not been evaluated in pregnant women.

A cutaneous myxoma, a.k.a. superficial angiomyxoma, consists of a multilobulated myxoid mass containing stellate or spindled fibroblasts with pools of mucin forming cleft-like spaces. There is often a proliferation of blood vessels and an inflammatory infiltrate. Staining is positive for vimentin, negative for cytokeratin and desmin, and variable for CD34, Factor VIIIa, SMA, MSA and S-100.

Clinically, it may present as solitary or multiple flesh-colored nodules on the face, trunk, or extremities. It may occur as part of the Carney complex, and is sometimes the first sign. Local recurrence is common.

Acral persistent papular mucinosis is a skin condition caused by fibroblasts producing abnormally large amounts of mucopolysaccharides, characterized by bilaterally symmetrical, flesh-colored papules localized to the hands and wrists.

Verruca plana, also known as a "flat wart", is a reddish-brown or flesh-colored, slightly raised, flat-surfaced, well-demarcated papule of 2 to 5 mm in diameter. Upon close inspection, these lesions have a surface that is "finely verrucous". Most often, these lesions affect the hands or face, and a linear arrangement is not uncommon.

TS is considered to be a benign dysplasia, although it can be disfiguring and is sometimes itchy. It is not known whether TS lesions have the potential to develop into cancer; while this outcome has never been reported, some polyomaviruses are oncogenic. The natural history of untreated TS is not known and no long-term studies of its progress have been performed. Improvement in immune function has been reported to resolve symptoms in some individual cases. Treatment with antiviral drugs has also been reported to improve symptoms, but only as long as treatment continues.

TS has been reported almost exclusively in immunocompromised patients, primarily organ transplant recipients on regimens of immunosuppressive drugs, and also in patients with hematolymphoid malignancies. Interestingly, as of 2016 there were no case reports in the literature describing cases of TS in patients with HIV-AIDS.

There is compelling evidence that trichodysplasia spinulosa is caused by a polyomavirus called trichodysplasia spinulosa polyomavirus (TSPyV) or "Human polyomavirus 8". There is evidence that exposure to the virus is common among healthy adults; estimates of seroprevalence (that is, prevalence of detectable antibodies against viral proteins) in immunocompetent adults range from 70 to 80% in different sample populations. TSPyV infects the skin, but viral DNA is rarely detectable there in asymptomatic individuals even if they possess antibodies to the virus indicating exposure. It is not known whether TS represents new primary infection or opportunistic reactivation of a latent infection.

A carnosity is a medical condition defined by any abnormal fleshy excrescence or tuberosity. Carnosity comes from the word carnose which means a buildup of flesh.

Angiofibromas are small, reddish brown or even flesh-colored, smooth, shiny, 0.1- to 0.3 cm papules present over the sides of the nose and the medial portions of the cheeks. They contain fibrous tissue.

Prickly heat can be prevented by avoiding activities that induce sweating, using air conditioning to cool the environment, wearing light clothing and in general, avoiding hot and humid weather. Frequent cool showers or cool baths with mild soap can help to prevent heat rash.

"Miliaria pustulosa" describes pustules due to inflammation and bacterial infection. Miliaria pustulosa is preceded by another dermatitis that has produced injury, destruction, or blocking of the sweat duct.