A fifty-two-year-old housewife knew her diagnosis. It was erythema multiforme. But “why,” she asked, “does it keep coming back?” It all started seven years ago when she was vacationing on an island off the coast of Mexico. She had numerous blisters, diagnosed as a reaction to insect bites. They slowly faded over the next month, only to reappear six months later, when her mouth also became ulcerated. This time it took two months for healing. A third attack was attributed to eating too much chocolate. Subsequent attacks were erratic, but in the last year there had been six distinct episodes.

Her close observation of these attacks taught her that each one was preceded by a cold sore on her lip, face, or nose. Invariably, seven to ten days later she would see her skin covered with small and large blisters. They were strangely floral in their patterns, with rings of purple giving them a target or iris look. Dermatologists had made the diagnosis of erythema multiforme, of the blistering (bullous) form.

But the diagnosis did not tell her, or any of her doctors, about the cause of her blisters, or suggest how she could avoid these disabling attacks. Erythema multiforme was only a name, used to describe the way the skin reacts to at least seventy different causes. What she had was a reaction pattern, but to what was she reacting? Could it be a drug? No, she had no common thread of any particular medication before each attack.