One of my player asked if his new Cleric (with the Pestilence domain) could voluntarily fail its saving throws against diseases. I know you can give up your saves vs spells but I do not know if you can give up your saves vs other effects that require saves such as diseases.
This from a guy using Ebola as his avatar. (That is Ebola, right? Or some other filovirus?)
Anyway, I'd say you can always give up your save versus any effect, because it's the simplest answer and stands a decent chance of producing some interesting and fun results. Sure, you could argue that diseases are fought off automatically by the immune system and should be different, but that just gives you the headache of having to make a list of what saves can be voluntarily failed and what saves must always be rolled.
And that's nitpicky, boring, and doesn't really make your game any better, so you might as well just extend the rule for willingly giving up your saving throw against a spell to saving throws against everything. Besides, I've often found that throwing some of the mechanical control back to the players can produce some really unexpected (and fun) results.
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it's amazing what kind of trouble some players will choose to get their characters into
ryan