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.
Long-term treatment is associated with squamous cell carcinoma (but not with melanoma). A combination therapy for moderate to severe psoriasis using PUVA plus acitretin resulted in benefit, but acitretin use has been associated with birth defects and liver damage. Systemic agents
Psoriasis resistant to topical treatment and phototherapy may be treated with systemic therapies including medications by mouth or injectable treatments. People undergoing systemic treatment must have regular blood and liver function tests to check for medication toxicities. Pregnancy must be avoided for most of these treatments. The majority of people experience a recurrence of psoriasis after systemic treatment is discontinued. Non-biologic systemic treatments frequently used for psoriasis include methotrexate, ciclosporin, hydroxycarbamide, fumarates such as dimethyl fumarate, and retinoids. Methotrexate and ciclosporin are medications that suppress the immune system; retinoids are synthetic forms of vitamin A. These agents are also regarded as first-line treatments for psoriatic erythroderma. Oral corticosteroids should not be used, for they can severely flare psoriasis upon their discontinuation.Biologics are manufactured proteins that interrupt the immune process involved in psoriasis. Unlike generalized immunosuppressive medical therapies such as methotrexate, biologics target specific aspects of the immune system contributing to psoriasis. These medications are generally well-tolerated, and limited long-term outcome data have demonstrated biologics to be safe for long-term use in moderate to severe plaque psoriasis. However, due to their immunosuppressive actions, biologics have been associated with a small increase in the risk for infection.Guidelines regard biologics as third-line treatment for plaque psoriasis following inadequate response to topical treatment, phototherapy, and non-biologic systemic treatments.