Ghosts

Apparently they've got ghosts over at the medical clinic. The houskeeping staff has been seen spraying the premises with some liquid out of a big bottle. It says Holy Water on the bottle, and they use it to keep the ghosties at bay.
Does it work?
Well, I can only speculate. If it works, is it because...
- it renders any ghosts invisible/makes them friendly?
- it is a general fact that all ghosts can't stand holy water?
- holy water only works on ghosts that are still or at least were once themselves believers?
- the person spraying the holy water is a believer?
- ghosts don't exist in the first place, so spraying stuff only gets rid of what isn't even there, which means we could smear strawberry jam into every corner to make the place whatever-we're-afraid-of-proof?
Well, I don't know. But they do frequently spray the medical clinic with holy water here. It's part of the routine. Take out garbage, mow lawns, get supplies, have lunch, fix roof, spray holy water.
By the way, what happens if one drinks holy water? Is that a way to save oneself from becoming i.e. a zombie or vampire after one dies? Or will it make one a 'good' ghost? Or will it prevent one from ever becoming a ghost at all?
Perhaps holy water doesn't have any powers, maybe it works like the magic potion in 'Asterix in Britain'. Holy water. It simply has to be more exquisite than plain water.