Children used to naturally die out due to their weakness, and only the strongest would survive to see adulthood before advent of the modern medicine. This resulted in individuals who naturally would have been selected out of the genepool, to continue existing, and even spread their faulty genes on to next generations - be it females or males.

In essence, the success of modern medicine has doomed us to slow degeneration of mankind.

Pre-modern medicine:
People had lots of children, but depending on the health of parents between 1/4th and 3/4ths died before seeing adulthood due to plethora of different complications found in the human condition. This created a sort of natural 'filtering' system that removed the weakest links out of the human race, while the healthiest individuals would propagate their genes further, making the human race healthier in the long run.

Post-modern medicine
People have fewer children, but even if they had many, the implications of modern medicine result in the fact that a lot of children with complications get to grow up and even spread their conditions into new generations. The 'filtering' system is absent due to the success of the modern medicine. This in the long run implies a 'detoriation' of the humanity because those 'non-healthy' individuals will reduce the health potential of 'healthy' individuals in the long run.

Any book that discusses the phenomen?

Now I know that the reason for the 'decline' is not just medicine, there's also modern sedentary lifestyle, processed foods etc. But I'd like to see it from the angle of medicine, although I wouldn't mind a perspective that takes everything else into account as well (if it's done well). Is there anyone with medical field that takes this under consideration?