Will improving medical cures finally kill off the human race?

Your question shows a lot of insight and wisdom. We are meddling and we don't know what we are doing. Medical scientists think that they are improving things by making us resistant to various medical ailments etc. But crucially, we, and not nature, have decided what physical attributes we should be aiming for. Over long periods of time, it is the very existence of illnesses that causes the body to create immunities. We are in such an all fired hurry, that we haven't got time to wait. What about the law on unintended consequences?