Lessee, I have 2 allergy fics, and I'm pretty sure there are more in my fic stash... Both of the two I can think of RN are mildly Destiel (which I conveniently ignored, since I don't ship it, haha).

Reflex (http://archiveofourown.org/works/4925362/chapters/11301409) by BlackCatRunning
Now that he's human, Castiel really doesn't like having to ask the Winchesters all the embarrassing questions that come from experiencing mortal life for the first time. But when it comes to his health, perhaps he should.

Sting Operation (http://zana-zira.livejournal.com/9991.html) by zana_zira
Castiel has loved bees for a long time. He never thought anything of it when he used to walk straight into the middle of a flower patch, covered in bees from head to toe. But that was when he was an angel. Now that he's human, he's going to learn very quickly how dangerous even one sting can be. Sam & Cas friendship, established Destiel. Warning: several lame bee puns.

Ooh, also, not human!Cas, so it's totally okay if you don't wanna read it, but plenty of sick!Cas:
Pride and Pestilence (https://www.fanfiction.net/s/5586875/1/Pride-and-Pestilence) by Celtic Amazon
The Winchesters and Castiel go hunting for the latest horseman to be summoned: Pestilence. But there are things the angel isn't telling them, and his pride may prove to be his downfall.

I'll have to hunt for some others at some point. I know they're around... Haha.

Obviously not op, but thank you anyway as I was craving some good Castiel whump. :)