The person was deemed "impure" (see tumah and taharah) during their affected phase and is ultimately treated by the kohen. However, it is more likely that this confusion arose from the use of the same Greek term for both conditions. The Greeks used the term lepra (λεπρα) for scaly skin conditions. They used the term psora to describe itchy skin conditions. It became known as Willans lepra in the late 18th century when English dermatologists Robert Willan and Thomas Bateman differentiated it from other skin diseases. Leprosy, they said, is distinguished by the regular, circular form of patches, while psoriasis is always irregular. Willan identified two categories: leprosa graecorum and psora leprosa.Psoriasis is thought to have first been described in Ancient Rome by Cornelius Celsus. The British dermatologist Thomas Bateman described a possible link between psoriasis and arthritic symptoms in 1813.The history of psoriasis is littered with treatments of dubious effectiveness and high toxicity. In the 18th and 19th centuries, Fowlers solution, which contains a poisonous and carcinogenic arsenic compound, was used by dermatologists as a treatment for psoriasis. Mercury was also used for psoriasis treatment during this time period. Sulfur, iodine, and phenol were also commonly used treatments for psoriasis during this era when it was incorrectly believed that psoriasis was an infectious disease. Coal tars were widely used with ultraviolet light irradiation as a topical treatment approach in the early 1900s. During the same time period, psoriatic arthritis cases were treated with intravenously administered gold preparations in the same manner as rheumatoid arthritis.
The condition may be mild until exacerbated by stressful life events, or symptoms may develop and slowly trend worse with time. Pathophysiology
Mast cell activation can be localized or systemic. MCAS can present with a wide range of symptoms in multiple body systems, these symptoms may range from digestive discomfort to chronic pain, mental issues as well as an anaphylactic reaction. Some examples of  tissue specific consequences of mast cell activation include urticaria, allergic rhinitis, and wheezing. Systemic mast cell activation presents with symptoms involving two or more organ systems (skin: urticaria, angioedema, and flushing; gastrointestinal: nausea, vomiting, diarrhea, and abdominal cramping; cardiovascular: hypotensive syncope or near syncope and tachycardia; respiratory: wheezing; naso-ocular: conjunctival injection, pruritus, and nasal stuffiness). This can result from the release of mediators from a specific site, such as the skin or mucosal tissue, or activation of mast cells around the vasculature. Diagnosis
MCAS is often difficult to identify due to the heterogeneity of symptoms and the "lack of flagrant acute presentation". Many of the numerous symptoms are non-specific in nature. Mast cell activation was assigned an ICD-10 code (D89.40, along with subtype codes D89.41-43 and D89.49) in October 2016.