Well I don't see how they can get poison, I mean I kept them for weeks now and I haven't added anything new other than brand named prepackaged substrate, and packaged rodent feed and water. If there was poison in the water I would think I myself would be quite sick as well.
I never use any pesticides or anything like that in my house at all and haven't for years.
What else could be potentially poisonous?
If I were to assume it is a disease, then the snakes should be fine?