In Doctor Who episode S10E4 Knock Knock, we encounter some mysterious bugs, named Dryads by the Doctor, that may be alien, or not (we really know very little about them, sorry).
They live in the walls of the house, or, they are the walls of the house. You know, in a "Part of the crew, part of the ship" kind of way. And they eat people by drawing them into the walls and then, presumably, slowly devouring them. So far, so good.
And in the end, it turns out that they are needed by the landlord to
keep his daughter alive (OK, OK, not really his daughter. Spoilers, spoilers), because long ago, when his daughter was dying, they saved her life. And they have been keeping her alive ever since.
But, how? What did they do? That wasn't clear at all, only that they did. Not a word about how they did it.