The survivors/mind controlled question

Is it explained how the mind controlled people were picked and how they survived?

From what I understand, most people were killed by the snow. Then some of the survivors were killed by the bugs- or maybe they werent? We see them cocooning and dragging people away- I assumed they were dead by that point, but maybe not? Was the army of mind controlled people the people who were cocooned by the bugs or was it something else?