On a December evening in 1997, four million Japanese children sat down to watch Pokemon. Four seconds of flashing red and blue light later, hundreds were convulsing on living-room floors. It became the most infamous television broadcast in history, and the strangest thing about it is how many of the victims had never had a seizure before, or since.

In a quiet farm town, teenage girls began to jerk, stutter and convulse one after another, with no fever, no lesion, no poison anyone could find. Doctors called it mass hysteria. The families called it something in the ground. Fifteen years later, the honest answer is still uncomfortable.

On 13 October 1917, an enormous crowd at Fátima, Portugal, reported seeing the sun spin and plunge toward the earth. Even an anticlerical newspaper printed it. No observatory on the planet recorded a thing.

In July 1518 one woman began to dance in a Strasbourg street. Within a month hundreds had joined her, and the city's official cure was to hire musicians. Five centuries later, no one can say what really happened.

It began with three schoolgirls giggling in class. Within months, an unstoppable wave of laughing, crying and fainting had shut down schools across an entire region — and doctors could find nothing wrong.

A century ago an epidemic put its victims to sleep, then locked the survivors inside frozen bodies for years. Doctors never found what caused it.

In September 1944, an Illinois town armed itself against a prowler spraying sweet-smelling gas through bedroom windows. Police never found him — because he may never have existed.

Five times it swept through England, striking down the young and the strong before the day was out. Then, after 1551, it disappeared for good, and no one has ever proven what it was.