Rash
A rash is a change of the human skin which affects its color, appearance, or texture.
A rash may be localized in one part of the body, or affect all the skin. Rashes may cause the skin to change color, itch, become warm, bumpy, chapped, dry, cracked or blistered, swell, and may be painful.
The causes, and therefore treatments for rashes, vary widely. Diagnosis must take into account such things as the appearance of the rash, other symptoms, what the patient may have been exposed to, occupation, and occurrence in family members. The diagnosis may confirm any number of conditions.
The presence of a rash may aid diagnosis; associated signs and symptoms are diagnostic of certain diseases. For example, the rash in measles is an erythematous, morbilliform, maculopapular rash that begins a few days after the fever starts. It classically starts at the head, and spreads downwards.

Differential diagnosis
Common causes of rashes include:

Food allergy
Medication side effects
Anxiety
Allergies, for example to food, dyes, medicines, insect stings, metals such as zinc or nickel; such rashes are often called hives.
Skin contact with an irritant
Fungal infection, such as ringworm
Balsam of Peru
Reaction to vaccination
Skin diseases such as eczema or acne
Exposure to sun (sunburn) or heat
Friction due to chafing of the skin
Irritation such as caused by abrasives impregnated in clothing rubbing the skin. The cloth itself may be abrasive enough for some people
Secondary syphilis
Poor personal hygieneUncommon causes:

Autoimmune disorders such as psoriasis
Lead poisoning
Pregnancy
Repeated scratching on a particular spot
Lyme disease
Scarlet fever
COVID-19 (see Symptoms of COVID-19 § Other)

Conditions
Diagnostic approach
The causes of a rash are numerous, which may make the evaluation of a rash extremely difficult. An accurate evaluation by a provider may only be made in the context of a thorough history, i.e. medications the patient is taking, the patients occupation, where the patient has been and complete physical examination.
Points typically noted in the examination include:

The appearance: e.g., purpuric (typical of vasculitis and meningococcal disease), fine and like sandpaper (typical of scarlet fever); circular lesions with a central depression are typical of molluscum contagiosum (and in the past, small pox); plaques with silver scales are typical of psoriasis.
The distribution: e.g., the rash of scarlet fever becomes confluent and forms bright red lines in the skin creases of the neck, armpits and groins (Pastias lines); the vesicles of chicken pox seem to follow the hollows of the body (they are more prominent along the depression of the spine on the back and in the hollows of both shoulder blades); very few rashes affect the palms of the hands and soles of the feet (secondary syphilis, rickettsia or spotted fevers, guttate psoriasis, hand, foot and mouth disease, keratoderma blennorrhagicum);
Symmetry: e.g., herpes zoster usually only affects one side of the body and does not cross the midline.A patch test may be ordered, for diagnostic purposes.

Treatment
Treatment differs according to which rash a patient has been diagnosed with. Common rashes can be easily remedied using steroid topical creams (such as hydrocortisone) or non-steroidal treatments. Many of the medications are available over the counter in the United States.The problem with steroid topical creams i.e. hydrocortisone; is their inability to penetrate the skin through absorption and therefore not be effective in clearing up the affected area, thus rendering the hydrocortisone almost completely ineffective in all except the most mild of cases.

References
External links

Guide to rashes on Medline Plus Medical Encyclopedia – includes photographs
Links to pictures of skin rashes (Hardin MD/Univ of Iowa)
Pictures of common skin rashes compared (Dermapics)