A plunger pump including an airtight bellows surrounding a portion of a drive shaft to protect the drive shaft and associated seals and/or bearings from harmful fluids that are being pumped. No counter-pressure fluid exists inside the bellows, other than air, and the bellows is sufficiently rigid to withstand the pressure of the pumped fluid without collapsing. The interior of the bellows is in fluid communication with the atmosphere, negating the need for a reservoir to accommodate varying quantities of counter-pressure fluid as the pump operates.