An ambulatory infusion pump assembly and housing therefore has a cover and a base providing an enclosed cavity. The system includes a liquid pump in fluid communication with a supply source of a liquid drug. The pump and supply source are received in the housing cavity. Further, the system includes a seal providing a liquid tight seal between the cover and the base. A through passage extends from the enclosed cavity external to the housing and an infusion tube extends through the through passage. A fluid tight seal is formed about the infusion tube. A vent is provided in the housing that maintains a balanced pressure state between the enclosed cavity and the atmosphere, while at the same time, the vent prevents the ingress and egress of contaminants into and fluid out of the enclosed cavity.