Some theories have it that dealing with disease was one of the major evolutionary pushes for sexual reproduction. The constant reshuffling of the genetic deck allows one's offspring to be less vulnerable to the same old diseases. The common cold may repeatedly knock you down, but it won't kill you because it, too, has to constantly evolve in order to both overcome the human immune system and the fact that all humans are just that tiny bit different.

Welcome to the Red Queen's Race. Here's an example of how it works from real life.

Of course, later on, we'll get to their over optimistic vaccination practices, and I say this as a fervent pro-vaxxer.

BTW, do you have an edition with Huxley's retrospective on the story and how he'd do it differently if he were to write it at the age he was writing the retrospective from?