An external infusion device enclosed in a multiple-housing casing. Enclosed within an outer wall, a first housing encloses a reservoir of liquid and a drive mechanism. A second housing encloses a pump and electronics for controlling the drive mechanism to dispense the liquid from the reservoir according to a selected pattern. A separate third housing encloses a battery or batteries. The first and third housings are vented to atmosphere via primary vents with hydrophobic barriers. The second housing is vented to at least one of the first and third housings via a secondary vent with a hydrophobic barrier. The hydrophobic barriers allow passage of air, and allow pressure equalization, but prevent passage of liquid. Liquid entering one housing is prevented from flowing into the other adjoining housings.
