In another topic, bestworking, some others, and I discussed why circumcision was needed. If I interpret her last post correctly, her question was not about sin causing disease and the need for circumcision, but why God did not equip man to face this if He knew it would be in his future.
Yes, I believe God is omniscient; however, I'm not sure that He chooses to look into the future in our lives, partially because of our free will. Most Christians would say that if young Joe married young Jane, God would bless their union, not desiring to see it end.
Yet, if Joe went to Afghanistan, and Jane suddenly decided she liked Jim...liked him very, very much, God would allow Joe to divorce Jane and be free to marry again. After some time, if Joe married single Jenny, then God would want Joe and Jenny's marriage to last forever. There are other scenarios that we could use, but this pretty much sums up the theory that God may choose not to know some things in our future.