Mankind has known since time immemorial how important a source of clean, fresh water is to a settlement. What we often fail to realize is why exactly are these things so important, and the advances of modern science finally give us the answer - because of disease. Thus what the Romans did finally acquires sense for the modern man, rushing to build yet another factory. But even more importantly, Sanitation shows that there is an invisible world of tiny life out there, which can and should be studied.