The Empty Child / The Doctor Dances

This whole story is so beautifully structured (and generally just amazing and brilliant) that I really don't want to complain: Captain Jack as the accidental antagonist, his nanogenes as the key to the whole thing, etc. But... does it make sense that the nanogenes recognise Nancy's DNA and use it to fix everyone? I'm really not sure. Yes, thematically it's an absolute triumph, and the nanogenes were seeded very early on in the first episode, but it makes me uncomfortable. Still. Amazing.