Why exactly were they living in a bunker anyway?

Humans have lived, thrived and survived for thousands of years in snow dominated regions of the earth.... so why were they hiding? Its not like it was a toxic or radiated environment like other post-apocalyptic films. There were whole cities to plunder and that's not including fishing under the rivers etc.

Also hiding your population from the cold naturally makes them susceptible to cold viruses, so its no wonder so many were dying. Again we are not talking about some alien virus..... the weak would have died off naturally and the 2nd to 3rd generation would have been fine.

Christ they didn't even have snow shoes! And why was everyone seemingly living in concrete, damp and cavernous factories/ industrial plants which would have been horrendously cold.