Etiology

We possess no certain knowledge as to either the proximate or remote causes of the disease. It is not uncommon to find an extensive eruption in those who otherwise appear to enjoy the most robust health; while, on the other hand, it may appear only during periods of temporary debility, as in women during pregnancy and lactation. That the affection is constitutional and connected with similar conditions to those underlying eczema we have no doubt, and each year's experience more strongly confirms this opinion. Some have claimed that the eruption is purely local or due to the presence of a parasite. Positive evidence of this is wanting. Others pretend that it is but a relic of syphilis handed down from a remote ancestor. This view also has little to support it. Psoriasis is often hereditary. It attacks males more than females, and is most common between the ages of fifteen and thirty. Persons of sanguineous temperament are most liable to the disease perhaps, and it is seen in persons of all classes of society, and mostly in summer and winter.