An infusion pump for infusing a medical treatment fluid intravenously to a patient includes a collapsible treatment fluid bag juxtaposed with an inflatable drive fluid bladder confined between a pair of opposing containment members which cause the bladder to impinge against the bag as the bladder is inflated. A drive fluid pump controllably inflates the bladder, thereby displacing treatment fluid from the bag into an outlet tube affixed thereto which conveys the treatment fluid to the patient where it is received intravenously. The treatment fluid flow rate through the tube is controlled by increasing or decreasing the drive fluid pump output and consequently the pressure in the bladder in response to a pressure sensor in fluid communication with the bladder. Alternatively, the treatment fluid flow rate is adjusted by opening or closing the outlet tube by means of an occluder or restrictor positioned along the tube.