u will be surprised how often large outbreak of parasites infections there are in "developed world's" public water supply: Sweden [1], US [2]. CDC even has a web page set up for Drinking Water-associated Disease Surveillance.
[1] http://www.ncbi.nlm.nih.gov/pmc/articles/PMC3966397/
[2] https://en.wikipedia.org/w/index.php?title=Water_supply§...