You would be surprised how resilient a human body and human will (its a necessary combination) is sometimes in dire situations. A lot of the prisoners of these camps were people who had very decent lives, more than decent sometimes. There were successful businessmen, doctors, lawyers and others who probably never experienced bitter cold like that in their life but they survived.

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I once had a similar experience (obviously nowhere near the extent of the holocaust survivors). When I was a kid I was a bit fat, and my teacher thought it would be funny to punish the fatty by making him run 4 laps of the football field in the scorching heat of an extreme New Delhi summer noon. I had completely exhausted everything by the 2nd lap, but I did it somehow, as the sadist bitch smiled at a distance.

Sometimes you discover things about your body in situations like that you never knew you had before.