A backflow-prevention system that substantially prevents contaminant backflowat a treatment site. The systemin-cludes a fluid ejector tube for insertion into the patient's mouth and abackflow prevention device that receives fluid from the fluidejector tube. The backflow prevention device includes an internally positionedvalve member. The valve member limitscontami-nant backflow. The valve member is movable between open and closedpositions based on a vacuum pressure condition in thesys-tem.
