Sitting straight up in bed at midnight exactly because what if the other kid was immune too. He had a hole in his knee. Not a bite mark. He was quiet, but not behaving erratically. We were given the impression he had walked a long way to the city, fainting from hunger/exposure within sight of the guards. The signs all said - nobody makes it past a day.
The light turned red, but it turned red for Ellie too. Because the cordyceps is there, but it's not hurting her. That kid could have just been exhausted, and we could've been dangled that hint specifically because it was never meant to be just Ellie. By keeping her a secret there was no reason for anyone to look for immunity in anyone else. We've always known it was statistically improbable for her to be the only one ever. Maybe it really is that everyone just kills anyone who's been infected before the immunity can be confirmed.
