When Aziraphale crouches down to her level, Melanie meets his gaze squarely. He's the one who doesn't understand, but how can she blame him? No one else here understands, either. Even after she's explained it so many times, she often gets the feeling that they don't completely believe her. Maybe they don't want to. She couldn't really blame them for that, either.

She shakes her head at Aziraphale, but it's a gentle, sympathetic gesture, not like the fervent denial of before. "I'm not a prisoner. I asked them to keep me here, because I'm not really a little girl." Since he seemed to have an easy grasp of what 'synthesizing proteins' meant, Melanie decides to just be up front and hope that he can follow her. "I'm a symbiote. My universe has a mutated strain of cordyceps - that's a kind of fungus - that learned how to infect people. My parents were infected before they had me, so now I'm… both. Human and Ophiocordyceps."

She studies him a moment, making sure he's keeping up. Then, her voice grave, she adds, "If it gets out into the world, it will wipe out everyone. That's why I have to stay in." A beat. "Do you understand?"