The letter1 from Warren, Pa., printed in this issue is a contribution to an important, if little understood, problem. On several previous occasions extensive outbreaks of intestinal disturbance have been reported in connection with the use of contaminated water. One of the most thoroughly studied of these cases is that recorded by the well-known British health officer, Dr. Thresh.2 In that epidemic, which occurred at Chelmsford, England, there were about 1,400 cases, mostly among adults, and fourteen deaths. The original source of the water supply was apparently not exposed to infection. The initially pure water, however, was passed into a small uncovered reservoir, through which it flowed to mix with water from a spring in a larger covered reservoir. The small reservoir was not bricked over above the ground level, and during a heavy rain water from the immediate surroundings could run directly into it. Adjoining one end