Abstract: A dental apparatus backflow-prevention system substantially prevents contaminant backflow into a patient's mouth. The system includes a saliva ejector tube for insertion into the patient's mouth and for removal of saliva, blood, etc. when a vacuum is applied. A backflow-prevention device receives fluid from the saliva ejector tube and includes an internal valve seat. Valve componentry disposed within the backflow-prevention device includes a housing and a seating mechanism operably supported by the housing to automatically engage the valve seat and prevent contaminant backflow upon release of the vacuum. Upon application of vacuum, however, the seating mechanism automatically disengages from the valve seat and allows fluid flow away from the patient's mouth. According to one embodiment, the seating mechanism is functionally flexible for movement within the housing away from the valve seat, and is biased to move toward the valve seat upon release of vacuum.