Any dermatitis may heal leaving pale skin, as may excessive use of corticosteroid creams used to treat episodes of eczema. The hypopigmentation is due to both reduced activity of melanocytes with fewer and smaller melanosomes.

The condition is most often seen in children between the ages of 3 and 16 years and is more common in males than females. However adults can also suffer from this disease.

It may occur more frequently in lighter-skinned patients, but is more apparent in those with darker complexions.

Up to a third of US school children may at some stage have this condition. Single-point prevalence studies from India have shown variable rates from 8.4%,

to 31%.

Other studies have shown prevalence rates in Brazil of 9.9%,

Egypt 13.49%,

Romania 5.1%,

Turkey 12% where higher rates were seen in those with poor socioeconomic conditions,

and just 1% in school children in Hong Kong.

Often no specific cause for the eruptions is found. However, it is sometimes linked to underlying diseases and conditions such as:

- Food (including blue cheese or tomatoes).

- Contact Dermatitis (i.e. cleaning agents, fabric softeners, etc.)

- Fungal, Bacterial and Viral infections such as sinusitis, tuberculosis, candidiasis or tinea.

- Drugs including finasteride, etizolam (and benzodiazepines), chloroquine, hydroxychloroquine, oestrogen, penicillin and amitriptyline.

- Cancer (especially the type known as erythema gyratum perstans, in which there are concentric and whirling rings).

- Primary biliary cirrhosis.

- Graves disease.

- Appendicitis.

- Lupus

- Pregnancy (EAC usually disappears/stops soon after delivery of baby).

- Hormone (Contraceptive Pill, Stress, Hormone Drugs)

- Lyme Disease

It is very rare and estimated to affect 1 in 100,000 per year. Because of its rarity the documentation, cases and information are sparse and not a huge amount is known for certain, meaning that EAC could actually be a set of many un-classified skin lesions. It is known to occur at all ages and all genders equally. Some articles state that women are more likely to be affected than men.

The patches of pityriasis alba may last from 1 month to about one year, but commonly on the face last a year. However it is possible that the white patches may last for more than 1 year on the face.

There is no known cause of this disease; There is some evidence associating it with Parvovirus B19.

Pityriasis lichenoides et varioliformis acuta is a disease of the immune system. It is the more severe version of pityriasis lichenoides chronica. The disease is characterized by rashes and small lesions on the skin. The disease is more common in males and usually occurs in young adulthood, although it has been seen in every age group and every race. It is possible for the disease to go into remission for short periods of time or forever.

Dr. W.A.D. Griffiths, from Great Britain, classified six forms of PRP in the early 1980s. At this time, the causes of PRP are still unknown and symptoms can be difficult to diagnose. Frequently, more than one medical professional will be consulted before an accurate PRP diagnosis is made.

Dermatologists have identified both an acquired form and an inherited form (familial) of PRP and have described them in medical journals. The acquired form usually shows a spontaneous or gradual remission of symptoms within several years although long-term symptoms may continue for years. The inherited form starts early in childhood with persistent long-term symptoms into adulthood.

Although most people who develop PRP are over age 50, individuals of any age, race, and nationality can be affected. Women and men seem to be equally affected.

In most patients, the condition lasts only a matter of weeks; in some cases it can last longer (up to six months). The disease resolves completely without long-term effects. Two percent of patients have recurrence.

Pityriasis lichenoides chronica (PLC) is probably caused by a hypersensitivity reaction to infectious agents such as the Epstein–Barr virus. Other infectious agents include the adenovirus and Parvovirus B19.

This skin disease commonly affects adolescents and young adults, especially in warm and humid climates. The yeast is thought to feed on skin oils (lipids), as well as dead skin cells. Infections are more common in people who have seborrheic dermatitis, dandruff, and hyperhidrosis.

Pityriasis rubra pilaris (also known as "Devergie's disease," "Lichen ruber acuminatus," and "Lichen ruber pilaris") refers to a group of chronic disorders characterized by reddish orange, scaling plaques and keratotic follicular papules. Symptoms may include reddish-orange patches (Latin: "rubra") on the skin, severe flaking (Latin: "pityriasis"), uncomfortable itching, thickening of the skin on the feet and hands, and thickened bumps around hair follicles (Latin: "pilus" for hair). For some, early symptoms may also include generalized swelling of the legs, feet and other parts of the body. PRP has a varied clinical progression and a varied rate of improvement. There is no known cause or cure although stress is a common factor.

It was first described by Marie-Guillaume-Alphonse Devergie in 1856, and the condition is also known as Devergie's disease.

Actinic keratosis is very common, with an estimated 14% of dermatology visits related to AKs. It is seen more often in fair-skinned individuals, and rates vary with geographical location and age. Other factors such as exposure to ultraviolet (UV) radiation, certain phenotypic features, and immunosuppression can also contribute to the development of AKs.

Men are more likely to develop AK than women, and the risk of developing AK lesions increases with age. These findings have been observed in multiple studies, with numbers from one study suggesting that approximately 5% of women ages 20–29 develop AK compared to 68% of women ages 60–69, and 10% of men ages 20–29 develop AK compared to 79% of men ages 60–69.

Geography seems to play a role in the sense that individuals living in locations where they are exposed to more UV radiation throughout their lifetime have a significantly higher risk of developing AK. Much of the literature on AK comes from Australia, where prevalence of AK is estimated at 40–50% in adults over 40, as compared to the United States and Europe, where prevalence is estimated at under 11–38% in adults. One study found that those who immigrated to Australia after age 20 had fewer AKs than native Australians in all age groups.

Pityriasis lichenoides is a form of pityriasis.

Types include:

- Pityriasis lichenoides et varioliformis acuta

- Pityriasis lichenoides chronica

If the condition thickens, turns red and irritated, starts spreading, appears on other body parts, or if the baby develops thrush (fungal mouth infection), fungal ear infection (an ear infection that does not respond to antibiotics) or a persistent diaper rash, medical intervention is recommended.

Severe cases of cradle cap, especially with cracked or bleeding skin, can provide a place for bacteria to grow. If the cradle cap is caused by a fungal infection which has worsened significantly over days or weeks to allow bacterial growth (impetigo, most commonly), a combination treatment of antibiotics and antifungals may be necessary. Since it is difficult for a layperson to distinguish the difference between sebaceous gland cradle cap, fungal cradle cap, or either of these combined with a bacterial infection, medical advice should be sought if the condition appears to worsen.

Cradle cap is occasionally linked to immune disorders. If the baby is not thriving and has other problems (e.g. diarrhea), a doctor should be consulted.

The parapsoriasis groups, described and debated for nearly a century, has spawned a confusing nomenclature. There are some authors who prefer to limit the term "parapsoriasis" to large- and small-plaque variants only. However, the following classification scheme is now generally accepted:

- Large-plaque parapsoriasis

- Small-plaque parapsoriasis

- Pityriasis lichenoides

- Pityriasis lichenoides chronica

- Pityriasis lichenoides et varioliformis acuta

- Lymphomatoid papulosis

The overall prevalence of PR in the United States has been estimated to be 0.13% in men and 0.14% in women. It most commonly occurs between the ages of 10 and 35. It is more common in spring.

PR is not viewed as contagious, though there have been reports of small epidemics in fraternity houses and military bases, schools and gyms.

Types include:

- Pityriasis alba

- Pityriasis lichenoides chronica

- Pityriasis lichenoides et varioliformis acuta

- Pityriasis rosea

- Pityriasis circinata

- Pityriasis rubra pilaris

- Pityriasis versicolor

- Dandruff, historically called "Pityriasis capitis"

- Pityriasis amiantacea

It is seen in:

- Albinism

- Idiopathic guttate hypomelanosis

- Leprosy

- Lleucism

- Phenylketonuria

- Pityriasis alba

- Vitiligo

- Angelman syndrome

- Tinea versicolor

- An uncommon adverse effect of imatinib therapy

In 2016, interferon gamma/CXCL10 axis was hypothesized to be a target for treatments that reverse inflammation. Apremilast is undergoing investigation as a potential treatment .

As PNP is ultimately caused by the presence of a tumor, it is not contagious. There is no known way to predict who will become afflicted with it. Patients with cancer are therefore a group at risk. Although PNP has been known to affect all age groups, it is more likely to afflict middle-aged to older patients.

Melanin is a pigment in the epidermis that functions to protect keratinocytes from the damage caused by UV radiation; it is found in higher concentration in the epidermis of darker-skinned individuals, affording them protection against the development of AKs. Fair-skinned individuals have a significantly increased risk of developing AKs when compared to olive skinned individuals (odds ratios of 14.1 and 6.5, respectively), and AKs are uncommon in dark-skinned African Americans. Other phenotypic features seen in fair-skinned individuals that are associated with an increased propensity to develop AKs include:

- Freckling

- Light hair color

- Propensity to sunburn

- Inability to tan

Assurances that this condition will clear as the baby matures are very common. However, studies have shown that the condition occasionally persists into the toddler years, and less commonly into later childhood. It tends to recur in adolescence and persists into adulthood. In an Australian study, about 15 percent of previously diagnosed children still had eczema 10 years later. Sometimes, cradle cap turns into atopic dermatitis. Rarely, it turns out to be misdiagnosed psoriasis.

Although other forms of the disease occur at younger ages, some individuals start having long term symptoms at thirty years of age. This disease also affects adolescents and young adults. This also affects the immune system which therefore results in rashes. The symptoms rarely affect the face or scalp, but occurs at other sites of the body. The duration may last for months or even several years. For instance, new crops of lesions appear every few weeks.

In contrast to cutaneous LP, which is self limited, lichen planus lesions in the mouth may persist for many years, and tend to be difficult to treat, with relapses being common. Atrophic/erosive lichen planus is associated with a small risk of cancerous transformation, and so people with OLP tend to be monitored closely over time to detect any potential change early. Sometimes OLP can become secondarily infected with Candida organisms.

Parapsoriasis refers to one of a group of skin disorders that are characterized primarily by their resemblance to psoriasis (red, scaly lesions), rather than by their underlying cause.

Neoplasms can develop from parapsoriasis. For example, it can develop into cutaneous T-cell lymphoma.

The word "parapsoriasis" was formed in 1902.