I did some testing:
In a siege, Leprosy
always hit the besieging army.
Nation A is besieging Nation B in province X.
Nation C casts Leprosy on province X: Nation A's army affected.
Nation B casts Leprosy on province X: Nation A's army affected.
And I thought this was interesting: Nation A casts Leprosy on province X: Nation A's army affected.
Ouch!
I also tested Manifestation: randomly hits a commander of A or B in province X. Does not matter who cast it.
Probably it's a good idea to test if you are unsure about other spells.