Fish feeding on city sewage in one location can transmit disease when caught and consumed from distant pure waters.

Original passage

In many places fish become so contaminated by the filth on which they feed as to be a cause of disease. This is especially the case where the fish come in contact with the sewage of large cities. The fish that are fed on the contents of the drains may pass into distant waters and may be caught where the water is pure and fresh. Thus when used as food they bring disease and death on those who do not suspect the danger.