Marian Monroe Oxford has long descended into madness. It is only thanks to Legion that she currently has a moment of clarity. It is also thanks to Legion that she is basically immortal.
The micromachines inside her body has a standing order not to let her die, no matter what. And after they had forced themselves into her mouth, sliding down her throat and pooling in her stomach, they proceeded to replicate and remodel some of her vital organs.
Doing so has allowed Legion to inconspicuously pump vitality liquids directly into her body and organs, thus mending any injuries she had suffered and regenerating any limbs she had lost.
The vitality liquid that has just been mentioned is actually purified Health Potion.