A system for reducing pathogens in a biological fluid such as whole blood or blood-derived products includes a pump configured to propagate the fluid through a serpentine-shaped flow path while exposing the fluid to UV irradiation. Extensive mixing of flow is accomplished by a pump control system programmed to cause the pump to vary the flow of fluid. In embodiments, the pump may be automatically operated to periodically switch the flow back and forth between a high flow rate and a slow flow rate. Alternatively, the pump may be periodically stopped or even cause the flow to reverse direction for short periods of time.
