Approximately 280 million people globally, 4% of the population, have difficulty with itchiness. This is comparable to the 2–3% of the population suffering from psoriasis.

The exact mechanism of the condition is unknown, though some studies have suggested the itching occurs in response to increased fibrinolytic activity in the skin. Later studies indicated inappropriate activation of the sympathetic nervous system may play a part.

The name is derived from Latin: aquagenic, meaning water-induced, and pruritus, meaning itch.

The more poignant part of this disorder is the lack of desensitization for water and aqua intile injection as allergen even on repeated exposure. Avoidance of allergen as a general principle in any allergic disorder necessitates the evasion of water exposure. Topical application of antihistamines like 1% diphenhydramine before water exposure is reported to reduce the hives. Oil in water emulsion creams, petrolatum as barrier agents for water can be used prior to shower or bath with good control of symptoms. Therapeutic effectiveness of various classes of drugs differs from case to case.

The exact underlying cause of aquagenic urticaria is poorly understood.

One way to treat xerotic eczema is to avoid ing the affected area and to apply anti-itch or moisturizing lotion frequently.

A study published in 2005 found positive results from soaking the affected area in water for twenty minutes and then applying mid- to high-strength corticosteroid ointment.

Itch (also known as pruritus) is a that causes the desire or reflex to scratch. Itch has resisted many attempts to classify it as any one type of sensory experience. Modern science has shown that itch has many similarities to pain, and while both are unpleasant sensory experiences, their behavioral response patterns are different. Pain creates a withdrawal reflex, whereas itch leads to a scratch reflex.

Unmyelinated nerve fibers for itch and pain both originate in the skin; however, information for them is conveyed centrally in two distinct systems that both use the same nerve bundle and spinothalamic tract.

The most common food allergies in adults are shellfish and nuts. The most common food allergies in children are shellfish, nuts, eggs, wheat, and soy. One study showed Balsam of Peru, which is in many processed foods, to be the most common cause of immediate contact urticaria. A less common cause is exposure to certain bacteria, such as "Streptococcus" species or possibly "Helicobacter pylori".

Chronic idiopathic hives has been anecdotally linked to stress since the 1940s. A large body of evidence demonstrates an association between this condition and both poor emotional well-being and reduced health-related quality of life. A link between stress and this condition has also been shown. A recent study has demonstrated an association between stressful life events (e.g. bereavement, divorce, etc.) and chronic idiopathic urticaria and also an association between post-traumatic stress and chronic idiopathic hives.

Drug-induced pruritus is itchiness of the skin caused by medication, a pruritic reaction that is generalized.

Xerotic eczema (also known as "Eczema craquelé", "Pruritus hiemalis", "Asteatotic eczema", "Winter itch", "Desiccation dermatitis," and "Winter eczema") is a form of eczema that is characterized by changes that occur when skin becomes abnormally dry, itchy, and cracked. Lower legs tend to be especially affected, although it can appear in the underarm area as well.

Xerotic eczema is common in elderly people, though it is not uncommon for people in their 20s. It can appear in red, bumpy, pimple-like irritations. Shaving can cause it to become inflamed.

This is a skin disorder characterized by a self-perpetuating scratch-itch cycle:

- It may begin with something that rubs, irritates, or scratches the skin, such as clothing.

- This causes the person to rub or scratch the affected area. Constant scratching causes the skin to thicken.

- The thickened skin itches, causing more scratching, causing more thickening.

- Affected area may spread rapidly through the rest of the body.

Many hypothesize LSC has a psychosomatic origin. Those predisposed to itch as a response to emotional tensions may be more susceptible to the itch-scratch cycle. It may also be associated with nervousness, anxiety, depression, and other psychological disorders. Many people with LSC are aware of the scratching they do during the day, but they might not be aware of the scratching they do in their sleep. LSC is also associated with atopy, or atopic dermatitis (eczema).

Lichen simplex chronicus (LSC) (also known as "Neurodermatitis") is a skin disorder characterized by chronic itching and scratching. The constant scratching causes thick, leathery, darkened, (lichenified) skin. This condition is associated with many factors, including the scratch-itch cycle, psychological stressors, and atopy. LSC is more common between ages 35 and 50 and is seen approximately twice as often in women compared to men.

Generally, lichen nitidus is asymptomatic and self-limited; therefore, no treatment is required. However, if persistent pruritus is present, or the appearance “...interferes with daily activities or outlook...” topical glucocorticoids may be tried. If the disease process is symptomatic, generalized and extensive, oral glucocorticoids may be indicated. Other reported treatments include PUVA, UVA/UVB phototherapy, astemizole, acitretin, and etretinate.

When appears with sun/humidity; air conditioning (cool dry air) reduces swelling and discomfort.

The Mazzotti reaction, first described in 1948, is a symptom complex seen in patients after undergoing treatment of onchocerciasis with the medication diethylcarbamazine (DEC). Mazzotti reactions can be life-threatening, and are characterized by fever, urticaria, swollen and tender lymph nodes, tachycardia, hypotension, arthralgias, oedema, and abdominal pain that occur within seven days of treatment of microfilariasis. The Mazzotti reaction correlates with intensity of infection; however, there are probably multiple infection intensity-dependent mechanisms responsible for mediating this complex reaction.

The phenomenon is so common when DEC is used for the treatment of onchocerciasis that this drug is the basis of a skin patch test used to confirm that diagnosis. The drug patch is placed on the skin, and if the patient is infected with the microfilaria of "O. volvulus", localized pruritus and urticaria are seen at the application site.

A case of the Mazzotti reaction has been reported after presumptive treatment of schistosomiasis and strongyloidiasis with ivermectin, praziquantel and albendazole. The patient had complete resolution of symptoms after intravenous therapy with methylprednisolone.

If a specific cause for pruritus ani is found it is classified as "secondary pruritus ani". If a specific cause is NOT found it is classified as "idiopathic pruritus ani". The irritation can be caused by intestinal parasites, anal perspiration, frequent liquid stools, diarrhea, residual stool deposits, or the escape of small amounts of stool as a result of incontinence or flatulence. Another cause is yeast infection or candidiasis. Some diseases increase the possibility of yeast infections, such as diabetes mellitus or HIV infection. Treatment with antibiotics can bring about a disturbance of the natural balance of intestinal flora, and lead to perianal thrush, a yeast infection affecting the anus. Psoriasis also can be present in the anal area and cause irritation. Abnormal passageways (fistulas) from the small intestine or colon to the skin surrounding the anus can form as a result of disease (such as Crohn's disease), acting as channels which may allow leakage of irritating fluids to the anal area. Other problems that can contribute to anal itching include pinworms, hemorrhoids, tears of the anal skin near the mucocutaneous junction (fissures), and skin tags (abnormal local growth of anal skin). Aside from diseases relative to the condition, a common view suggests that the initial cause of the itch may have passed, and that the illness is in fact prolonged by what is known as an itch-scratch-itch cycle. It states that scratching the itch encourages the release of inflammatory chemicals, which worsen redness, intensifies itchiness and increases the area covered by dry skin, thereby causing a snowball effect.

Some authorities describe “psychogenic pruritus” or "functional itch disorder", where psychological factors may contribute to awareness of itching.

Ingestion of helminth (worm) "Enterobius vermicularis" (pinworm, or threadworm) eggs leads to enterobiasis, indicative of severe itching around the anus from migration of gravid females from the bowel. Severe cases of enterobiasis result in hemorrhage and eczema.

Psychogenic pruritus is a common manifestation of chronic anxiety, usually a localized itch, especially in the anogenital area.

The condition is often managed with drugs including H1-antihistamines, tricyclic antidepressants, tetracyclic antidepressants, selective serotonin reuptake inhibitors, antipsychotics, or benzodiazepines.

Riehl melanosis is a form of contact dermatitis, beginning with pruritus, erythema, and pigmentation that gradually spreads which, after reaching a certain extent, becomes stationary. The pathogenesis of Riehl melanosis is believed to be sun exposure following the use of some perfumes or creams (a photocontact dermatitis).

HIV-associated pruritus is a cutaneous condition, an itchiness of the skin, that occurs in up to 30% of HIV infected people, occurs when the T-cell count drops below 400 per cubic mm.

Bullous pemphigoid may be self-resolving in a period ranging from several months to many years even without treatment. Poor general health related to old age is associated with a poorer prognosis.

Shiitake mushroom dermatitis (also known as "flagellate mushroom dermatitis", "mushroom worker's disease", and "shiitake-induced toxicoderma") is an intensely pruritic dermatitis characterized by disseminated 1mm erythematous micropapules seen in a linear grouped arrangement secondary to Koebnerization due to patient scratching. It is caused by the ingestion of shiitake mushrooms and was first described in 1977 by Nakamura. Although it is rarely seen outside of China and Japan due to a lower incidence of shiitake consumption outside these regions, there is a well-established association between flagellate dermatitis and shiitake mushroom (Lentinula edodes) ingestion. Bleomycin ingestion may also cause similar findings. On physical exam, one key difference between the two is that post-inflammatory hyperpigmentation changes are usually seen with bleomycin-induced flagellate dermatitis and are not typically present with shiitake mushroom induced flagellate dermatitis. The median time of onset from ingestion of shiitake mushrooms is typically 24 hours, ranging from 12 hours to 5 days. Most patients completely recover by 3 weeks, with or without treatment. Although the pathogenesis of shiitake induced flagellate dermatitis is not clear, the theory most argued for is a toxic reaction to lentinan, a polysaccharide isolated from shiitake mushrooms. However, Type I and Type IV allergic hypersensitivities have also been supported by the 24-hour median time of onset, clearance in 3–21 days, severe pruritus, benefit of steroids and antihistamines, and lack of grouped outbreaks in people exposed to shared meals containing shiitake mushrooms. Most cases reported shortly after its discovery were due to consumption of raw shiitake mushrooms, but several cases have since been reported after consumption of fully cooked mushrooms.

Very rarely seen in children, bullous pemphigoid most commonly occurs in people 70 years of age and older. Estimated frequency is seven to 14 cases per million per year, but has been reported to be as high as 472 cases per million per year in Scottish men older than 85. At least one study indicates the incidence might be increasing in the United Kingdom. Some sources report it affects men twice as frequently as women, while others report no difference between the sexes.

Many mammals can be afflicted, including dogs, cats, pigs, and horses, as well as humans. It is very rare in dogs; on average, three cases are diagnosed around the world each year.

Autoimmune estrogen dermatitis presents as a cyclic skin disorder, that may appear eczematous, papular, bullous, or urticarial. with pruritus typically present, skin eruptions that may be chronic but which are exacerbated premenstrually or occur immediately following menses.

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".

Adverse reactions to biologic agents, such as imatinib, occur in more than 80% of patients, and can be characterized by edema and pruritus without dermatitis.