I'd rather see an other option:
50% chance to cutting the palgue in half with de discovery of medicine.
The Spannish Flu is the real last plague, but was never cured. Smallpox is destroyed but is took a long time.
It would be nice to end those damn plagues though, but would that be a little to easy?
Well, we have AIDS, we have hepititus B, cancer, all on the rise. I believe AIDS is a true epidemic, even though our society doesn't fear it like we did so long ago. when the medieval plagues happened, they weren't understood, and now that we understand the plague comes from many things, and not from a god, we can better withstand them.
Between nanotechnology, genetics and tampering with evolutionary viral strains, we are actually in danger of making worse plagues to weaponize. With the advent of medicine, our societies are able to respond better than we could at those earlier ages.
As far as in-game, i would think that there should be no way to end plague. in a size 20 city, even with health improvements, we can assume there are still factors to breed diseases, and spread them. the game has no way to account for health care systems. some systems like in america, are not designed to cure you - only to provide you "long term treatment" to keep you as a financial investor in the medical system as per debt slavery.
even in a future-tech society - who is to say we will have a utopia where industry exists as a means of improving our species and habitat? Can we really say that money will stop being the primary motivator? Even if the medical industry is permanently reformed with a new direction to focus ONLY on public health and not on making money - who is to say we will be able to cure plagues even then?
there are still types of sickness we can not cure, natural and manufactured. example, radiation poisoning or the leftovers of biological weapons.
Therefore, because humans will probably never see an age where we live without plague - the game should never see an age or tech that ends it altogether.