The present invention generally relates to occluders and, in particular, to occluders for use in medical infusion systems. In one aspect, the occluder contains one or more tubes through which fluid can flow, and the occluder can be automatically controlled to allow or at least partially prevent fluid flow. In some cases, the occluder may be constructed and arranged to fail (e.g., due to loss of power) in a “closed” position, i.e., a position that at least partially prevents fluid flow. In addition, in one embodiment, the occluder may be manually overridden, even when in a failed state, by a user. For instance, in the event of a power failure, a user may be able to manually override the occluder to remove tubing from the occluder. Such occluders may find use, for example, in medical devices such as hemodialysis machines, medical infusion pumps, plasmapheresis devices, and the like.