Is modern medicine affecting the human gene pool?
Not in any significant measure up till now by the measurements of accepted medicine as far as I can see. There is a quite a lot of arguments for the opposite for quite a lot of reasons by quite a few groups. Are they ignored or suppressed? I guess it's up to the educated individual to decide on what they believe.
Is curing hereditary diseases on a genetic level a scientific possibility?
I don't think we're to the point where science can with absolute certainty design genetic cures. From what I've heard from the scientists we can basically make changes and see the direct results, but we don't know if or what other consequences there are.
If so, why there’s a focus on supressing those diseases or their symptoms?
How do you mean? If it's a disease, it's something bad - And a cure, or at least something that eases the symptoms, is something good no?