The answer is going to depend upon your theology. Christians with a strong view of original sin and a certain literal understanding of Gen 3, are going to say that everything wrong with the world is a result of original sin. God designed a world where everything would work together, no person on animal would suffer. Original sin affected not just us but animals and even inanimate things. There are two reasons for this: Genesis 3 suggest that God actually changed the way things worked, cursing the ground. But more generally, man was designed to be the ruler of the world, and when the ruler is unable to do his job, the whole creation suffers.
Many Christians today don't accept this kind of view of original sin. They may believe, as some have posted here, that there was death and suffering before the fall, and even that Adam and Eve didn't literally exist, so the fall isn't an event in history, but a way of talking about human rebellion against God. Thus when you ask about questions like this you won't get a theological answer, but a scientific one: why would it make sense that bodies would work in such a way that this suffering occurs?
Christian Forums has people with all of these views. It seems to be a matter of luck which group you run into on a particular question. The folks who have answered here (including me) don't hold the most literal view of the Fall. It seems that conservative Christians are most active in creating web sites, so it's not surprising that what you've seen is different. To get views more in tune with critical scholarship there are smaller numbers of web sites, at least in part because preparing well researched content is a lot of work. In the case of evolution and biological issues, biologos.org is a good place to start. Here's a series on this subject from that site:
http://biologos.org/blog/death-and-pain-in-the-created-order-part-1. It will give you a broader perspective of the range of views on where pain came from, and the theological assumptions behind those views. (Please do look at all 4 parts of the series. There's a link at the bottom of the page "Next post")
You may find, however, that any theological treatment of suffering leaves you feeling unsatisfied. If so, you are not alone. Job had the same problem. His friends presented all the answers, but none really worked for him. His real answer came from experiencing God directly. In my opinion, God has not created a world where everything is free of pain. The biologos series suggests why that might not be the best way for creatures, intelligent or not, to develop. Remember that this is a God whose response to sin and suffering was to join us and experience it himself, and bring us through it with him.
The problem I have with answering everything by original sin is that it gives us a view of God that I find kind of unbelievable. Would God really allow one person to put him in a position where all of creation isn't the way he wanted it? Didn't he create the garden, knowing what would likely happen? It seems to me that we have to assume that creation as it is now is in accordance with God's intentions, and deal with it as is. Even if he chose to use Adam and Eve and their sin as part of getting it where it is now.
