On The Turn. (Scenes) Chief: One of those...things stung me. But I haven't turned into one of them like the others did.
Cortana: Maybe it's because you're a good man, Chief...
"Chief...perhaps it's because of how our neural networks are linked together, the Flood could not gain a foothold into your nervous system?"
or
"I believe that you were a mere puppet Chief, until I unleashed my countermeasures device and neutralized the Flood's neural network"
That's right -- Cortana is the hero, Chief is merely a high-tech Mercedes.