Etiology

In a person predisposed to this affection almost any external irritation may cause an outbreak; and an exactly similar eruption may be provoked in some by contact with certain poisonous plants, as the common nettle. More frequently, however, the eruption is of inter­nal origin, and is but the reflection of pre-xisting irrita­tion of the gastro-intestinal or sexual organs. Certain articles of diet - as crabs, lobsters, various shell-fish, and certain fruits - as well as certain medicinal agents, excite a gastro-intestinal derangement, which is chiefly made manifest through the cutaneous disturbance.

Under the first head, or local excitants, rank the acarus, fleas, bugs, mosquitoes, lice, flannel, the contact of numerous other irritants, such as "jelly-fish."