The body shivers when it's in a state of mild hypothermia.

[He gives her a pointed look. Do you see what you're doing right now, Murphy?]

When it ceases to shiver, you'll be in an advanced state. Stay by the fire. You won't be of any use to me if you lose consciousness or become frostbitten. I understand your wanting to help, Lieutenant, but 'helping' in this case means being smart and acknowledging your own limitations.