It's hard for me to disassociate the sadness of the "Whatever Happened..."' story itself from the sadness in knowing it's the last we'd see of Curt Swan and Julius Schwartz Superman.
I think I was gonna die from total despair when first reading "the Last Days of Superman" when I was wee one. I just start reading and now Superman's dying!!! And even his friends from the future know it and help out in his greatest deeds to save Earth???
There were two big "Virus X" plotlines in Superman (plus a lesser one where Supergirl also gets it). There's the much-anthologized "The Last Days Of Superman", where it doesn't turn out to be Virus X after all, and "The Leper Of Krypton", where it is indeed Virus X. The latter is arguably a sadder tale. Superman is put in a coffin and gets thrown into the heart of a sun to be cremated. Poof! Ahh... someone did a writeup on it:
http://www.fortunecity.com/tatooine/niven/142/profiles/pro05.htmlAs an aside, when I read the Alan Moore Superman-Swamp Thing story when it came out, I went into a Snit Supreme thinking "Why doesn't Superman solve this virus the same way he did with Virus X? Alan Moore must not really grok Superman". <laughs>