Well, if a bad-sewage-sickness DID happen, how likely is it to stay put among the people that caused it instead of spreading?
I suspect that it's not the sort of thing that then stays put.
Sewage related sicknesses are in almost all cases common source epidemics. In non-medical terms, what this means is that you get x, say cholera or e. coli, from the source, drinking water or food from a contaminated field. You don't get it from others. So, unless the water from the Amish property is seeping into the general water supply or the Amish are selling their crops to others, no, the sickness is highly unlikely to spread. In fact, I'm having trouble coming up with a sewage related sickness that doesn't function as a common source epidemic though I am sure there must be some.