As Sarah and Jay struggled up the face of what would eventually be called Donner Pass on December 17, 1846, they knew nothing of hypothermia or hyperthermia. But under all the layers of sweat-soaked wool in which they were swaddled, their bodies were already fighting a silent, internal war between death by fire and death by ice, swinging back and forth between thermal extremes in a way that threatened to destabilize their regulatory systems and their bodies’ precious reserves.
