Incapacitation

When a squad member takes enough damage, they will be incapacitated. There are several states of Incapacitation represented by a squad member having less than 0 Constitution (CON), and most of them become progressively more severe if left untreated.

Revival
For an incapacitated squad member to survive, they must be healed using the Surgeon skill and a Surgeon Kit. If only knocked Unconscious, a squad member may gain the Mending status and will eventually get back up if left unharmed.

Death
If a squad member is left untreated or sustains enough damage, they will die. Death in the wasteland is permanent, and that squad member's skills and abilities will all be lost for good. You will, however, at least be able to recover all the items and equipment they were carrying.