What I recall from my days attending church with relatives is that people only think holy water protects them from spiritual ills, but mostly they don’t think about it at all. It’s almost like a low-level communion. Same goes for lighting votive candles in front of a cross or image. It’s a purely symbolic “sacrifice,” and nobody I ever knew thought there was any kind of mojo in the image, or that any spiritual being appreciated the candles. It’s just sort of fun and gets people in that mystical mood. Just one more thing religious people do on auto-pilot.

So first one needs to determine whether, theologically, disease is evil (harm actively intended by malevolent beings) or just an ordinary hazard of living. Was holy water ever intended to be effective against “plain old biology”?

Then there is the question of how “imperviousness to common ailments” would work. Would holy water actively repel pathogens, forcing them to stay on the original victim or propelling them through the air to a nearby wall or floor or some other out-of-harm’s-way surface (taking care, of course, that they not be inhaled by anyone); and what would be a safe place, anyway? Or would it cover pathogens in some kind of protective seal and sequester them at the bottom of the font for later removal during routine cleaning? Or would it actively kill biological pathogens (bacteria, viruses, prions, parasite eggs, etc.) and actively disassemble non-living pathogens (poisons)?

In the latter case, holy water would have some powerful magic since it would be, effectively, a perfect universal antiseptic.

It is not clear what believers think holy water protects them from, but their behavior suggests that they do not expect it to protect against the routine physical threats of the real world. Being “supernatural in substance”, apparently it operates only against supernatural dangers.

Render unto God that which is God’s and render unto Pasteur and Lister that which belongs to Medicine.

It’s not like there’s any misuse going on. This water is literally becoming something toxic simply by virtue of someone using it in the manner for which it was ordained. There’s nothing sinister at work here, just plain old biology.

Therefore, if this is supernatural in substance, shouldn’t it have some sort of imperviousness to common ailments?

This is a little like the instruction on the mayonnaise label: Refrigerate after opening. Once the customer has stuck his or her grubby fingers in it, the manufacturer is not going to vouch for its continued purity.

And one which has some interesting implications of its own.

Considering its actual function, that resemblance could lead to some entertaining, if disgusting, mistakes.