Evolution doesn't have a direction, it's an explanation of a thing that happens. Diversity in the gene pool is almost certainly a plus for the long-term surviv

Evolution doesn't have a direction, it's an explanation of a thing that happens. Diversity in the gene pool is almost certainly a plus for the long-term survival of the species, but it's marginal, must like everything that would determine the long-term survival of the species. Almost all diseases that happen after a species' reproductive age is very difficult to select out either in a positive or negative way unless it happens to be strongly genetically linked (either through sharing genes or being very close on a chromosome) to a different positive or negative trait. Even then in the long run the survival of that trait will be dominated by its independent characteristics. The way to flip this around is that you could also look at the "survival" of the disease, disorder, etc. Not that we WANT these to survive, but they either will or won't. If a disease causes death 100% of the time at age 2 and provides no other benefits in the carrier / recessive trait it tends not to last in the gene pool for very long.