Backflow and Cross-Connection Are All That's Required
Where backflow occurs and cross-connections are present you have all of the necessary elements for contamination of the plumbing system and subsequently contamination of the public water system:
For a backflow to occur you must have a link to the water system (cross-connections) and some kind of force present (a backpressure or backsiphonage).
Example of Contamination
For example, suppose an automatic lawn sprinkler system is spraying a lawn when all of a sudden a backsiphonage occurs due to a fire truck pumping water or water rushing from a broken water main.
The resulting backflow from the lawn sprinkler system will flow into the plumbing system and then into the water distribution system. As the water backflows it can suck contamination into the lines through the sprinkler heads such as insects, pesticides, herbicides, fungicides, fertilizer, worms, and other contaminants.
Once in the distribution lines the contamination could go anywhere in the public water system.