If a microwave oven doesn`t leak microwaves, then it won`t affect such people at all. However, if microwaves do leak from a particular microwave oven, they will cause undesirable currents to flow in the electric leads of the pacemaker. That`s because a microwave consists of electric and magnetic fields, and an electric field exerts forces on charged particles. The mobile charged particles in the pacemaker`s electric wiring will experience these forces as the microwave encounters them and they will move back and forth with the microwave`s fluctuating electric field. The pacemaker`s wiring isn`t meant to carry these unexpected current flows, and the pacemaker and/or the person attached to it may experience unpleasant effects. While such problems are very unlikely, it makes sense to warn pacemaker users whenever a microwave oven is in use.