Natural selection is still active now, but it's just presenting itself in nastier ways as our medicine is getting better. We'll never be able to prevent all diseases - nature will never allow it.
Not realistically anyway, but that still doesn't make mmj_uk's point valid. Some people that are more susceptible to disease X could be protected if enough people took advantage of modern medicine when appropriate. Does it matter if the human race has lived through diseases before? The whole herd immunity concept is about preventing the spread and suffering of the disease not just to the people that get the vaccine, but also the people that don't. If the people that can take the vaccine do it'll save people that can't for whatever reason.
This was the exact problem with the MMR. When the scare happened parents wouldn't let their children get vaccinated. The rate dipped below the herd immunity rate so viruses like measles began to actively spread again instead of dying out. The end result was that several un-vaccinated children died, and you can be fairly confident that they wouldn't have contracted measles if the herd was immune.


