Actually, this is a fairly intelligent question, because what we are really doing with medicine is not strenghthening our bodies, but rather simply wearing body "armor", in the form of medicines and operations.
Is Western medicine somehow corrupting the gene pool?
No, I don't believe it is. I believe what has happened is that the medical world has advanced so rapidly over the last 1000 years that the advances in the gene pool nowadays look like a standstill. Sort of like comparing a car going 300mph to one going 1, the slow one doesn't even look like it's moving, but it is.
I feel that medicine will actually strengthen the gene pool, example being vaccines. Diseases like Polio and Smallpox killed thousands, then we got vaccines, and our bodies (thanks to the Medicine) have become immune to them. In this way, the medicine actually does increase "good side" of the gene pool, as our bodies have now become immune to the dieases. Perhaps in 5000 years time, we won't even need a vaccine.......
Things like Influenza and colds medicines may have a detriment on the gene pool, as we have become accustomed to the medicines and such. But advances in life, food, diets, and medicine have strengthened our pool.
I think people today, given training, swords, and shields, would kick the crap out of a Roman legion any day of the week, and twice on sundays. They didn't get the proper nutrition, the proper clothing, or the proper medicines to stay in peak fighting condition, whereas people today do. Therefore, the gene pool is strengthened by these things, evident in the fact that 2000 years ago, my height (5'3), would be considered GIANT, whereas today I'm a shrimp.
Interesting question though, well done.