Rad Child (Fallout: New Vegas)

Rad Child is a Fallout: New Vegas and Fallout Tactics perk and a Fallout: Brotherhood of Steel skill.

When you get any sort of radiation sickness, your health regenerates, the effect becoming more powerful with every threshold reached. However, radiation will continue to build up in your system, and can still kill you at the fatal threshold as normal.

Fallout: New Vegas
The health regeneration kicks in when the player gains radiation sickness and becomes more powerful depending on the severity:
 * Minor (200 rads): +2 health per second
 * Advanced (400 rads): +4 health per second
 * Critical (600 rads): +6 health per second
 * Deadly (800 rads): +8 health per second

You will still die when you reach 1000 rads. This perk does not heal limbs.

Fallout: Brotherhood of Steel
With this skill, Cain restores HP whenever he takes radiation damage. To take it, the player must be Level 10.