Miss Emily told us it's how they live as long as they do and how they never suffer diseases. They replace their organs with organs from us Donors and think nothing of it.

..We're not, though. I mean, we are. We should be. But they don't see us that way. 'Creatures,' Madame called us. 'You poor creatures.'

I don't know who the Previous are. It's the Originals that use the organs.