Odds: whatever disease spreads, some individuals resist it. The most probable hypothesis is that he's somewhat a partial zombie himself, but that he didn't died in the process and his brain wasn't damaged. But he's enough of a zombie to not "smell" (for the lack of another word) as a human anymore. Consider it as an "aftermath" of the zombie disease for him, despite he survived it. It could have been a permanent brain damage due to the fever, making him a "stupid human" who would be eaten alive the next day or die from starvation/dehydration within a week, but he was lucky.
He however can't be sure to be the only one...
No, human CELLS.
HUGE spoiler:
Tokiko never ate brain, and she's becoming near intelligent in its ending while she's clearly underfed, compared to the intelligent zombies. You see that too with the zombie harem, who "eat" only semen, dead skin cells and sweat.
Maybe check
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: currently 11 chapters, 42 planned. I'm waiting for the next chapters, too. You also have a link to the novel above (but the lack of proper layout makes it difficult to read).
