"But it's probably wrong to infer which water-borne infections were present four thousand years ago - they alter so quickly. And there's E. coli as well."

Broadly speaking I agree, but we know through remains and droves that large numbers of sheep were on the downs, and sheep means wool, meat, and cryptosporidum whether now or then. Whether this particular bug caused more illness than any other is impossible to say of course, but I lstill like the idea that they figured they had to keep the water as pure as possible.

vbb