It was fairly long. Tony gets very sick, and a room in his home is turned into his hospital room, basically. Hank McCoy frequently comes and tries to treat him, acting as his doctor. Hank Pym also pops in and out. I think it was Tony/Steve, but it mayyyyyy have been Tony/Bruce. Tony gets sicker and sicker, and eventually starts to get better, slowly.
It might have been Marvel-616, or at least had elements of that (such as Hank Pym!).
I enjoyed the heck out of it, and would really love to read it again, if you have any suggestions as to what fic it might have been.