shangrilada has a great, on-going Sam!asthma 'verse (http://shangrilada.livejournal.com/7345.html), and mentholpixie has a series where younger Sam suffers from epilepsy (just hit her 'epilepsy' tag.) None of them are slash, though.

Thank you soooo much! :)

I read this not long ago and thought it was pretty good. In it, Dean has diabetes. Westcott Preparatory Academy (http://www.fanfiction.net/s/3549161/1/Westcott_Prepatory_Academy) by adder574

Yes thanks I think I've read it before but this story is great!!

I second shangrilada's asthma!Sam 'verse! I also have several Dean-centric fics bookmarked:

wave obscura has an excellent 'verse wherein Dean has Cystic Fibrosis: Because of Houdini (http://wave-obscura.livejournal.com/15876.html) and its sequel Because of the Amazons (http://wave-obscura.livejournal.com/44578.html)

Perfectly Functional Member of Society (http://spn-hurtcomfort.livejournal.com/5443.html?thread=211523#t211523) is a commentfic where Dean is diagnosed with narcolepsy. (Scroll down for the fic - there's also another fill closer to the top).
S’okay. He’s My Brother (http://rainylemons.livejournal.com/17204.html) is a post-series fic where Dean has Chronic Fatigue Syndrome.
The Damned Lie In Fire and In Snow (http://rainylemons.livejournal.com/41828.html) is primarily a case-fic, but Dean has Reynaud's in it.

Finally, several epileptic!Dean fics:
Demon Blood, Sweat and Tears (http://ayslin.livejournal.com/28470.html)
Fallen Idol (http://nong-pradu.livejournal.com/14619.html)
Five Minute Warning (http://teashopmuses.livejournal.com/11539.html)
Of Guilt and Bonding (http://neonchica.livejournal.com/8061.html)

Wow thank you can't wait to read them! *hugs*

Sweet Surrender (http://sinnerforhire.livejournal.com/364293.html) (Jensen with diabetes)

Under Control (http://spn-hurtcomfort.livejournal.com/222308.html?thread=1313892) (Jensen with epilepsy)

Take The Hit (http://roque-clasique.livejournal.com/133634.html) (Dean with asthma)

Thank you!!! :D *hug*