Anti-reflux valve with environmental barrier

A medical valve including a housing having an inlet and an outlet and a normally closed disc valve, valve actuator, and valve seat situated therebetween, the disc valve being openable by a blunt cannula inserted through the inlet and acting upon the valve actuator, the disc valve being mounted for movement by the valve actuator away from the valve seat within the housing to create an uninterrupted annular through opening between the valve seat and the disc valve, and an environmental barrier separate from the disc valve and extending completely across the housing inlet, the environmental barrier including an elastomeric slit septum, the slit being normally biased closed and being openable under pressure of the blunt cannula to allow the blunt cannula to pass through the slit to the disc valve.