Jamie,
'semck: "The people in this painful universe have all sinned, and according to Christian doctrine, do deserve punishment"
'What was my sin, please? I know what Adam and Eve's sin was, but what sin did I commit? I wasn't present in the garden of Eden. My free will did not cause me to accept an apple from a snake. It wasn't my choice. It wasn't my act. How is it my sin?'
As I answer this, I also want to clear up a misconception. Not uncommonly (unfortunately) I have been somewhat unclear despite rambling on at shocking length. Specifically, you referred earlier today to "semck's view that humans are forced to suffer due to original sin, and that my illness is an example of such suffering." This, along with the above, makes me think I haven't stated my point well.
When I said that humans "do deserve punishment" because they have sinned, I was responding specifically to a point putin had made accusing me of utilitarianism. I was making the point that the suffering we are subjected to is not morally unjustified -- but I was not saying that it is necessarily caused by a desire to punish us. (That may or may not be part of it, depending on circumstances; we can't necessarily know).
To take an analogy, consider a youth who commits a crime and is sent to a reform school by the courts. Now an ordinary youth could not be sent to reform school by the courts, however good an idea it might seem. It would be wrong for them to step in and interfere in his life that way. But this youth committed a crime. He could have been sent to prison if they'd wanted, and so by sending him to reform school, they're well within their rights, and actually being nicer than necessary. That said, their hope in sending him there is probably not mostly to punish him -- it's to reform him and make him a better person.
So my point to putin was not that the _purpose_ of our suffering is punishment for sin, though it is partly that I suppose (much as the reform school is in some ways a punishment). The _moral justification_ for our suffering is our sin -- God has the right to subject us to it because He has the right to punish us much worse, to withdraw life altogether. But the _purpose_ could be much better things: improving us, teaching us lessons as Draug said, in some odd way hopefully drawing us closer to Him, making us realize out dependence on Him so that in the end, we will turn to Him and suffering will end.
The actual specific purpose, as FlemGem and others have pointed out (also I in my initial post, on the book of Job), we cannot know. What we can know, and believe as Christians, is that God has a morally good reason for it, that it will work out for ultimate net good, and that it will work out for ultimate specific good for those who do follow Him.
I hope that clarified my position some. Now, to your question.
"What was my sin, please? I know what Adam and Eve's sin was, but what sin did I commit?"
In my statement to putin on moral justification, I was actually referring to your sin, not Adam's sin. According to Scripture, all (except Christ) are sinful. This, it is true, is because of Adam's sin -- after he sinned, our nature was corrupted so that we are born with broken and sinful wills, in rebellion against God. Why it is that this trait would be inherited is an interesting question, and I don't fully claim to know. (It would certainly be an odd world if it weren't, and only some had fallen, of course!) Anyway, the point is that, born with this nature, you have sinned.
I don't know you and I can't tell you specifically what you have done. As FlemGem (I think) said, read Matthew 5-7 and you'll probably think of something. Meanness to another, jealousy, unjustified anger, selfishness (not just as a way of life, but even in an instance), unfairness, lust, worshipping gods (including material gods) other than God, disrespect of parents, etc., etc. Many of these sins seem trite to us because, well, we're all sinful, we all do it. The Bible makes it clear that they are anything but trite to God. He views them extremely gravely. They must eventually be punished, yet at the same time, He desires to forgive us and to draw us to less sin and to greater love. This is the central Christian message, the reason Christ did come to earth and suffer horribly.
Whether original sin itself would justify subjecting you to a fallen and painful world is an interesting question. It's also one I haven't studied or thought about sufficiently, but in this event, you have yourself sinned, and it was those actual sins of you yourself that I was referring to.
I hope I have addressed your question. Please do let me know if I have not, or if I should address something else. My own lack of clarity through verbosity is something I've become all too familiar with.