Agreed. Reducing the complexity of the human experience to "medicine" was a bit reductive on my part, although it certainly plays a part.

Evolution isn't a perfect system. It's a game of repeated "good enoughs". If the genes with a disadvantage get a little lucky with their reproductive odds during the initial generations of the mutation, there's no reason a fully detrimental mutation might not stick around for a while.

Also, humans tend to be the exception to a lot of rules. Our knowledge of medicine (Edit: and agriculture, and a buncha other things) significantly alters our fitness odds, and allows many genes to reproduce that may not have otherwise survived.