So, I have this trap. And it needs to hurt anyone that comes in contact with it (player and NPCs).

I can work out if people are in contact with it, that is no problem. My brain just must be dead or something, because I can't think of a way to damage people's health. I don't want to use a spell (since I don't want the magic effect visuals), and ModAv/SetAv on health doesn't work the same for NPCs as it does the player (I think). There must be a simple way to damage health that I'm not thinking of (I tried using the method that Traps ingame use, but that requires Havok collisions which I'm not using). The health needs to just get lowered and come back naturally (just like being injured in battle)

Any help would be appreciated.

Last time I checked, ModAV does work on health as expected, causing reversible damage. But I have my doubts, since the damage on other attributes by ModAv called from a script is irreversible instead.
If you don't mind using OBSE, you can use ModAV2 (which technically has been deprecated by ModAVMod damage) to be on the safe side.