[foolscap]

If I were trying to contaminate water supplies, I would use botulinum toxin, which is far more potent and cannot be removed by simple purification procedures.

I believe that would not work well in water because the microbe that produces it can only flourish under anaerobic conditions.

[CholeraJoe]

Toxin, not the organism. Purified Botulinum toxin only takes a gram to kill millions of people.