>- but how does it (zapper) distinguish between the good and bad stuff in your blood?
Actually, the zapper does not distinguish but there is a difference that allows the zapper to kill certain microbes but not cells in higher organisms.
Most microbes, including bacteria, viruses, fungi, protozoa, etc have a cell wall that is different in structure and function than the cell membranes that exist in many higher organisms. These cell walls tend to be rigid and will fracture under vibration whereas the cell membrane is very flexible (like rubber) and just quivers under the same vibration.