This invention provides a prefilled selectively activatable body-worn infusion-pump assembly for rapid delivery of large volumes or highly viscous volume comprising a housing； a pre-filled aseptically-sealed flexible drug reservoir-containing assembly； a conjoined, coordinately controlled and perpendicularly arranged cannulated needle dispensing and needle insertion assembly operationally connected to and in fluid connection with the pre-filled aseptically-sealed drug reservoir-containing assembly, wherein the cannulated needle dispensing assembly projects generally perpendicularly to a generally planar surface of the housing, promotes insertion of the cannulated needle in skin of a subject and promotes retraction of the needle within the cannulated needle dispensing assembly thereafter, and wherein the needle insertion assembly projects generally in a parallel orientation to a generally planar surface of the housing, which projection initiates opening a fluid path with the pre-filled aseptically-sealed drug reservoir-containing assembly； an engine assembly contained in the housing operationally connected to the pre-filled aseptically-sealed flexible drug reservoir-containing assembly promoting release of a drug contained therein, wherein the engine assembly comprises: a motor； a worm gear, operationally connected to the motor； a lifting gear, operationally connected to the worm gear； a piston operationally connected to the lifting gear； and a chassis fitted with an attachment promoting a floating connection thereto with the worm gear； wherein the worm gear and motor are mounted radially with respect to the chassis； a printed circuit board (PCB) assembly, which mechanically supports, electrically connects and controls the function of at least the engine assembly； and a single step activator that when engaged simultaneously promotes activation of the needle insertion assembly opening a fluid path with the pre-filled aseptically-sealed drug reservoir-containing assemb
