Let's say that a wizard casts possession on a bugbear. Now, this bugbear has spent some time in questionable places and has contracted both Filth Fever and Mindfire, the first being a disease causing Dex and Con damage, the latter being a disease causing Int damage.
The wizard is still possessing the bugbear when the diseases have their next ability damage 'tick'. Who gets the ability damage: the bugbear or the wizard?
The second paragraph of the possession spell description reads as follows (Emphasis mine):
If you are successful, your life force occupies the host body. The host’s soul is imprisoned with you, but can still use its own senses (though it can’t assert any influence or use even purely mental abilities). You can communicate telepathically with the host as if you shared a common language, but only with your consent. You keep your Intelligence, Wisdom, Charisma, level, class, base attack bonus, base save bonuses, alignment, and mental abilities. The body retains its Strength, Dexterity, Constitution, hit points, natural abilities, and automatic abilities. A body with extra limbs doesn’t allow you to make more attacks (or more advantageous two-weapon attacks) than normal. You can’t activate the body’s extraordinary or supernatural abilities, nor can you cast any of its spells or spell-like abilities.
Because of this, I assume that only the body (the bugbear) would get the Dex and Con damage from Filth Fever, though I can't find any rulings on this. But what about the Int damage from Mindfire?
Who takes the Int damage? The wizard, the bugbear or maybe even both?
I find this a difficult question because there are two minds in the same body, unlike similar possession spells like magic jar which removes the original soul from the body.
On a related note (and if this should be a separate question, I apologize):
How would spells like feeblemind work when targeting a body inhabited by more than one mind?