An acute inflammatory self-limiting condition of the skin and/or mucosa. It was first described by Ferdinand Ritter von Hebra, an Austrian dermatologist, in 1866. It is characterized by red macules (patches), papules (pimples), and sub-dermal vesicles (blisters) which develop into characteristic target lesions on the skin. Oral lesions are characterized by blood-filled blisters on the lips and intra-oral ulceration. The condition varies in severity from mild to severe (Stevensen–Johnson syndrome) and may be recurrent; up to 70% of these are triggered by a preceding herpes viral infection. Other cases are associated with infections, collagen disease, drug sensitivities, allergies, and pregnancy.