Cholera Dough
In the towns, the problem's considerably worse. The accepted means of waste disposal is just to chuck one's effluent in the river. Everything ends up there: fish guts from the markets, slop-bucket and chamberpot contents from the taverns, and worst of all -- the tanneries. Slaked lime, concoctions of mixed animal dung, tannic acid; tanners use some pretty vile chemicals when preparing leather. Worse yet, in a fantasy city you might get alchemists, and who knows what they might throw out? Bacteria and toxins abound in polluted rivers. Drinking water from these sources carries a far worse risk than dysentry: cholera!
Cholera: Cholera is a bacterial infection caused by the microbe Vibrio cholerae. The initial symptoms of cholera resemble dysentry: diarrhoea, muscle pains, etc -- but the symptoms are more pronounced, leading to vomiting, dehydration, circulatory problems and often death.
Infection DC: Ingestion 18
Incubation: 1d3 days
Damage: 1d6 Str, 1d6 Con
Additional Effects: When damaged, the character must succeed on another saving throw or 1 point of damage is permanent drain instead.
Cholera's chief advantage from a humiliating-the-players perspective is its speed and lethality. Once someone's infected, there's a big demand for a cure -- and if the water supply's infected, a whole lot of curing might be called for. Perfect for adding a bit of tension to the game, really.