Megalopolis and micro-organism

The story of Dr John Snow’s investigations into the causes of the cholera epidemics in mid-Victorian London has been written up several times, most recently in a book by Sandra Hempel which I reviewed in these pages six months ago. So do we need yet another account of them? Perhaps not, except that Steven Johnson approaches them from an unusually interesting angle: his book is less concerned with medical history than with urban history and, in particular, the rise of mega- cities.