A backflow-prevention system that substantially prevents contaminant backflow at a treatment site. The system includes a fluid ejector tube for insertion into the patient's mouth and for removal of fluids when a threshold vacuum pressure is applied. A backflow prevention device receives fluid from the fluid ejector tube and includes an internally positioned valve. The valve prevents contaminant backflow upon release of the vacuum pressure or a reduction in the vacuum pressure. Upon application of vacuum pressure, the valve moves into an open position to allow fluid flow away from the treatment site. The system can also include a disposable cover member that extends over at least portions of the fluid ejector tip, the backflow prevention device, and other features such as an ON/OFF valve assembly of a suction system. Similar principles of backflow prevention can be applied to fluid dispensers (e.g., liquid soap dispensers) wherein backflow of the fluid into the fluid storage container is undesirable.
