What's the actual trick that nearly eliminates infant mortality? Through all of recorded history, about half of infants die before their first birthday, but starting in Europe in the 19th century and continuing in the third world today, region after region seems to experience spontaneous revolutions where one generation sees 50% of babies die and the very next generation sees less than 10% of their children lost.

So what was the magic "this kills a shitload of babies" problem plaguing humanity for millennia and what is the simple, recently discovered fix for it?