I’m writing this on the balcony of my hotel room, looking out over a wooded glade and, beyond that, to the bare, rocky outline of a high ridge of the Bavarian Alps.

Birdsong echoes among the trees – the only sound in this Arcadian valley apart from the regular plash of a lone swimmer doing backstroke lengths in the outdoor pool on a terrace over to my right. Most of the rest of the guests in this idyllic retreat are probably lounging in one of two enormous spas which are located in different parts of the site.

It has got me thinking about the extraordinary surge in the popularity of spa breaks in recent years. Not that we should be surprised at our seemingly voracious appetite for health treatments,...