Since 4 October 2001, scores of children at two-dozen schools across the United States have been breaking out in a mysterious, itchy rash. Most of those afflicted are girls - 62 out of 67 in Medford, Oregon, for example. Although the rash quickly disappears with no after-effects, schools have been closed and parents have held emergency meetings.
Dermatologists, epidemiologists, environmentalists, even scientists at the Centers for Disease Control have tested for viruses, pesticides and mold spores, with absolutely no results. One doctor checked out glitter gel, stick-on tattoos, fruity lotions and other gear beloved of pre-adolescent girls. Some of the kids suspect allergies to the local tree frogs or the cafeteria food. On the web, conspiracy theorists blame aeroplane chemical trails, pet gerbils and the drinking water. Anxious parents have wondered if there could be links to bioterrorism.
In an article about the cases in the New York Times Magazine last month, the journalist Margaret Talbot speculates that these cases are incidents of mass sociogenic illness. Moreover, the story's alarming headline - "The Post-9/11 Mystery Rash" - hints at links to post-traumatic anxiety about the World Trade Center attack, or the anthrax scare that followed.
Talbot is correct in identifying the rashes as episodes of psychological contagion, or what is less euphemistically called mass hysteria. Such acute, short-term somatic reactions are extremely common, and mystery rashes are a well-known psychogenic phenomenon. The skin is extremely vulnerable to emotion and stress, as seen in blushing, flushing, hives or eczema - and itching, like yawning, is a notoriously suggestible symptom. In addition, in the past century, more than 60 per cent of cases of mass hysteria reported in English-language medical journals have taken place in schools - closed environments where there may be academic pressures, and where seeing another person fall ill creates anxiety and sympathetic symptoms.
In March 2000, when fourth-grade girls in Rhode Island developed rashes, Dr Robert Settipane told parents that the cause was probably stress over maths assignments. They were outraged: "How could I say such a horrible thing? How could I say their children were hysterical?" Parents believed their daughters were allergic to the maths textbook and had elaborate tests carried out on it.
Girls and women are a high-risk group for psychogenic illness. In the Belgian Coca-Cola panic of July 1999, for example, teenagers claimed to have been poisoned by drinking Coke. Many were hospitalised, and Belgium and France briefly banned all Coke products, although no contaminants were discovered. Out of 75 cases, 72 were girls. In a celebrated English case, mobs of schoolgirls collapsed at a Nottingham jazz festival when they thought a farmer had sprayed pesticides. Perhaps, as Gary Small, a psychiatrist at UCLA suggests, girls are "more likely to talk to each other about symptoms and how they're feeling".
Moreover, threats of bioterrorist attacks can have psychological effects on a civilian population. There were many such panics last autumn, involving both males and females, children and adults - and not just in the US. In Manila on 3 October, 1,000 students were treated for vague symptoms rumoured to be due to bioterrorism.
But it is highly unlikely that this recent American outbreak is related to the events of 11 September, except in the mind of New York Times headline writers.
There were kids with mysterious rashes last year and there will be kids with mysterious rashes next year. But they may have a lot more to do with Euclid than with Osama.