Nowhere is this more true than with portable electronics, an area in which medical electronics is proliferating. ESD upsets can be caused by either direct or indirect discharge. Direct discharge occurs only to metallic members of the equipment, while indirect discharge can occur to a nearby metallic member, with electric or magnetic field coupling completing the circuit.
Good shielding is fairly easy to accomplish when metal enclosures are used.
However, most modern electronic equipment has plastic enclosures, which are much more difficult to shield adequately. So the best defense is to prevent discharge from occurring in the first place. It is important to consider ESD when designing enclosures, because that is when steps can be taken to minimize discharge. ESD-proofing an enclosure after the design stage can be difficult. Regardless of any other EMC issues dealt with in the equipment's design, ESD considerations must be specifically addressed.