I would like to see a city where pipes are guaranteed leak free, for example by making them double walled with high pressure air in the outer layer, and then seeing if disease levels in the city are lower.
Contamination rarely happens outside of the source of supply, and not somewhere along the pipeline.
Not much, because the water mains pressure keeps nasty things from entering the pipe.
However, when the system is depressurized due to a power outage or due to running out of water, nasty things can happen (stuff entering the water pipes, oxygen from air bubbles causing rust), and that's why after such events boil-off orders are issued for a few weeks afterwards until it can be reasonably assumed that all pipes have been flushed and all air bubbles have gone.
Considering the difficulty and cost of repairing underground anything, most of which will be there for many many decades, it's never going to be perfect, but there's a lot of resources that do go into improving this.
I can tell you factually a lot of work goes into measuring leakage, narrowing down what part of the water system it is coming from (most active components are metered in some way, and you can use math to determine where all of the water is not making it through a segment), and correcting those issues where it is cost-effective to do so.
This kind of stuff is typically death by a thousand cuts.
Add on that a lot of the places it leads are under roads that will have to be shut down for weeks/months and you start to realize the costs and impact of fixing these leaks are enormous.
Followed by ecosystem being collected and put back into drinking water, most of which only has pretty lightweight treatment which doesn't even involve testing for any viruses which have snuck through.