This episode of More or Less was broadcast on Thursday, 12 June, 2003 on BBC Radio 4 at 15:00 BST.

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When it comes to disease and illness, either you've got it or you haven't, you might think.

Counting the number of cases of an illness matters, especially when it comes to something as deadly as the Sars virus.

Yet in Canada, more cases emerged after it was thought all had been detected and contained.

But the problem of what epidemiologists call "caseness" - when is it a case of one thing rather than another - bedevils the measurement of illness.

In More or Less this week, we examine the problem and highlight one of the most startling statistical curiosities in medicine: the dramatic rise in cases of autism.

Here, the figures have prompted some to talk of an autism epidemic, while others put them down to simple changes in reporting.

The numbers of reported cases do tell us something serious is going on. But what?

Also this week, with the North Pole conquered on foot and the conquest of Everest recalled, we tell the story of the explorer who believed that numbers expressed the divine in nature.

But his search through environmental extremes for the expression of mathematical purity in the world brought about a collapse of his scientific - and religious - faith.

Producer: Michael Blastland
Editor: Nicola Meyrick