Stevens-Johnson syndrome or erythema multiforme is a skin disorder due to an allergic
reaction or infection.
The exact cause is unknown. The disorder is believed to involve damage to the blood vessels of the skin, followed by damage to skin tissues.
Some forms of this condition are more severe than others.
Erythema multiforme minor is not very serious. Most erythema multiforme is caused by herpes simplex or mycoplasma infections.
Erythema multiforme major is more severe, and is known as Stevens-Johnson syndrome. The more severe form is usually caused by reactions to medications, rather than infections.
Erythema multiforme occurs primarily in children and young adults.
Stevens-Johnson syndrome at Wikipedia