This is a fic I read ages ago:
It started with Sam having to choose between John and Dean and John was the one that ended up dead. Dean end up pretty "pissed" at Sam(John aand Dean were shouting why Sam should let the other one live).
They they end up going on a hunt about a mysterios sickness. Sam is feeling realy guilty and starts to hallucinate, wich he thinks because how bad he feels about his dad. Dean has been previously really unsupportive and they later find out that Sam has the sickness.

It might have been on ff.net but you never know.

I Apreciate all and any help!