An infusion unit has a flexible container and an open-ended pipe having a first opening in communication with the container and a second opening axially opposite the first. The pipe has smaller and larger diameter portions with the smaller diameter portion located between the second opening and the larger diameter portion. An axially moveable plug is receivable within the smaller diameter portion to seal the second opening of the pipe. An axially moveable engaging member with an end for engaging the plug and a needle at the opposite end for piercing the stopper of a vial. The plug may be pushed by the engaging member from the smaller diameter section to the larger diameter section so as to permit flow through the pipe between the first and second openings to the vial and container.