Erythema multiforme (EM) is usually a reaction of the skin and mucous membranes that occurs suddenly.

[1] It appears as a symmetrical rash and may include the mucous membrane lesions.

This means that the body is sensitive to something that causes the skin and mucous membranes to react.

Evenly distributed bumps on the skin become classic iris or target lesions.

The cause of EM appears to be a highly sensitive reaction that can be triggered by a variety of causes.