I've traveled to India and it seemed like I was always sick. Population is over a billion, everywhere is crowded, open sewers, eating with your hand, for a start. This chart for India makes perfect sense because disease flows freely in India. It is a grand experiment to see what happens naturally in an epidemic.

The proof that herd immunity works is that humans exist. If herd immunity didn't work, we would have died off before we could ever form villages, much less cities and intercontinental travel.