Typhoid fever is such a widespread and universal disease that any observation tending to show its origin or mode of conveyance has more than ordinary interest. Drinking water has long been suspected as one of the media by which typhoid bacilli enter the body, and there set up their characteristic sequelæ. This suspicion has been more or less strengthened from time to time by epidemics in which there was considerable probability that an infected water supply was the means by which cases of typhoid fever multiplied. In the epidemic of typhoid fever, which forms the basis of this paper, a chain of evidence of cause and effect is so complete that it is worthy of presentation. In order that the whole situation may be as clear as possible it will be necessary to relate what might be called a history of the water supply.