Question: What is backflow?
Answer: Backflow happens when clean water reverses direction, pulling (through suction) dirty water into the water supply. More specifically, backflow happens when three conditions are met: a direct or indirect cross-connection (between the drinking water system and the fixture where a potential water contaminating hazard exists), a hydraulic condition (backpressure or backsiphonage), and a hazard substance.
Problems happen when the backflow preventer breaks or is compromised and your drinking water is contaminated because the preventer no longer prevents contaminants from entering your water source. Next step: sickness because of contaminants. It's serious enough that it is now a legal requirement that all owners of irrigation systems must have their backflow preventers tested annually.
Backflow is serious and causes significant health problems if not addressed. Learn more about backflow testing.