Well Poisoning Accusations in Medieval Europe: 1250-1500
In late medieval Europe, suspicions arose that minority groups wished to destroy the Christian majority by poisoning water sources. These suspicions caused the persecution of different minorities by rulers, nobles and officials in various parts of the cont...
They don't even need to. Destroying all the baths and hygiene systems from the Roman era will do the work.
Oh and the Jews are the ruling class, it's called the Jesuits/Vatican. And they'll find people to blame.