http://www.watts.com/pdf/F-sbn.pdf

Bottom line: When there's a drop in pressure in a city water system, all kinds of bad stuff may be siphoned into the system through you're neighbor's plumbing.

Case studies begin on page 6. Best not read before lunch.

Lots of focus on how the proper backflow preventer (which the company sells) could have prevented the incidents. Unfortunately, there's not much you can to to protect yourself from everybody else.