So what happened between TC and CD that conclusively proved he did not have the infection or cured it? (since it cant just be Rashid getting a good enough look at Harry with his 'magic bug detecting eye' to determine whether he's infected-he could've done it earlier)
Well, Harry died and came back, for one thing. Rashid might have decided, that however he did it, the infection would have died out or left through that process. Maybe that's exactly what happened, and the gatekeeper scanned Harry twice, and now he is clean. Maybe Rashid needs the gate plus his eye in order to see if someone is infected. So so many questions.
Hmm, I actually wanted to say something about headaches that occurred in the scene with Rashid, but then I got sidetracked. Still, both things are probably important.
Here is what I originally wanted to say about headaches:
I am kind of convinced by now, that the headaches started in camp kaboom, and I think they are suppressed memories of the sight.
In WN, we have:
Oh, God.” She sighed, smiling. “Yes.”
I muttered under my breath, rubbed at the incipient headache beginning between my eyes, and thought dark thoughts. Dammit all, every time I’d opened myself up to some kind of horrible psychic shock in the name of investigation, I’d gotten another nightmare added to my collection. Her first time up to bat, and the grasshopper got…
Something like New Mexico. Jesus. I didn’t want to think about that. I rubbed at the fresh headache sprouting between my eyebrows.
The first one is Harry thinking about the sight and the things he's seen with it in general, the second one is when he is starting to think about a specific thing that happens. Both times he gets sidetracked by the headache, and he drops the topic.
If you compare it with the scene in CD with Rashid:
"Experience," he said. "Decades of it. The Sight can help, but..." Rashid hesitated. I recognized it instantly, the hiccup in one's thoughts when one stumbled over a truly hideous memory gained with the Sight, like I'd had with--
Ugh.
--the naagloshii.
It seems kind of familiar. Thinking about something, and suddenly you are stopped in your tracks by the memory. So maybe the headache is to prevent Harry from remembering something. Maybe Lash put up a defense against those memories. Maybe Harry even asked her to, because it was too much for him to bear, whatever happened. I can't find it right now, but isn't the part where he is going to the cave to find the rest of the ghouls kind of a blur to him?