http://www.uesp.net/wiki/Oblivion:Damage_Attribute
I know, used that page to verify it but thought it might be possible in some obscure way like catching a disease.
I just tested the weakness to poison spell against mended flesh elementals from SI. Boss level enemy with 25% spell reflection and it worked extremely well.
A pretty complex spell; turn undead level 25 for 30 seconds, light 27 seconds, weakens to poison 100 in 5 sec, weakness to magic 100 in 5 sec, shock damage 3 for 1 second.
Now the turn undead does not work on the flesh elemental but would work with liches. Light to get a timeout warning of the turn undead, pretty pointless because the poisoned enemy dies fast, 5 seconds to indicate that the weakness to poison effect is active had been better.
Weakness to poison for poison effect, weakness to magic to boost it, yes the weakness to magic is dangerous however I always summon a clanfear or something to draw attention.
Shock damage is just so I know if the spell has been reflected.
I use a poison with silence, fire damage and damage health, work well enough, more so because I usually cast the spell a cople of times at enemy before hitting so the weakness to magic should boost the weakness to poison.
Enemy dies in seconds, I use blade of woe as it’s unenhanced and sits in my inventory so I can have it ready poisoned.