That was my thought as well. I'm not sure what the likelihood of a disease contaminating the entire resivoir is, but regardless wouldn't the water be sterilized before it hits the tap? After all it's exposed to other wildlife.
The disease would have to be transmitted via urine, then it would have to survive in mostly clean water for what... days? Months? Years? and then it would have to be effective after normal consumption. I'm not aware of any disease that would be transmitted this way.
There are occasionally outbreaks associated with municipal water supplies. I remember a big one from Milwaukee in the early '90s, looking it up, it was caused by a protozoa, cryptosporidium. They can be pretty durable:
