Spoilers below for the most recent Walking Dead episode, titled "The Big and Scary U."
Season 8 of The Walking Dead has not been much of a joyride for Father Gabriel, who was left stranded within the walker-swarmed Sanctuary by Gregory, and soon found himself sharing confessions with Negan. When we last saw him, he was a shivering and sweaty mess inside one of the Sanctuary's cells, causing many fans to wonder how he managed to get so sick in such a relatively short amount of time. Some, like me, figured a walker bite had to be involved, but one theory points to another cause that seems obvious in hindsight: he's suffering from radiation poisoning.
Throughout these early Season 8 episodes, The Walking Dead has seemingly set up its story to deal with a big chemical threat of come kind. It was at the Saviors' chemical plant outpost that Ezekiel's Kingdom crew got slaughtered, and it was outside that location that we saw all those upturned and emptied barrels, with assumed toxic waste spilling out into the small creek and surrounding area. Considering those barrels have likely been there for a long while, as well as the fact that walkers have been blindly bumbling through that poisonous glop for as long as it's been there, it's certainly possible (within the generally non-scientifically strict parameters of The Walking Dead) for there to be hundreds or thousands of contaminated walkers roaming the areas surrounding the Sanctuary.
If that's the case, then it's within the realm of possibility that whenever Negan and Father Gabriel covered themselves in affected walker guts in their attempt to get back to the Sanctuary's main building, the latter had adverse reactions. At this point, we know that characters who get a bit of walker blood in their system don't turn into zombies, but we're not at all sure what happens whenever toxified walker blood enters the equation. Just by touching it and putting it on his skin, Father Gabriel may have unwittingly poisoned himself. We technically didn't get to witness all of his problems whenever Eugene went to check on him, but radiation poisoning does cause symptoms to show fairly quickly, and Father Gabriel could very well have been nauseous and suffering from headaches and a fever, causing him to sweat buckets.
As one Redditor pointed out when sharing this theory, it's possible (and even likely) that Negan and the Saviors have no clue about the toxic waste that's possibly making all those roaming walkers even more dangerous. But Negan does seem to know that going all gut-covered for walker strolls is something that can make people sick, though he might just think it's to do with walker blood in general.
I almost hope this poisoning angle is indeed what's happening with Father Gabriel, since it would be highly disappointing if The Walking Dead didn't follow through on the toxic waste dumping in any interesting ways beyond showing off what Greg Nicotero's talented effects team is capable of. As well as making fans dream about the show bringing Shiva back as a radioactive zombie. (Call it Chekov's Toxic Waste Dump.) Not that I necessarily want Father Gabriel to suffer this kind of fate, and I definitely don't want to watch a future Walking Dead season where all the characters are afflicted with cancer, but I just want to see something interesting happen with all this.
Knowing how The Walking Dead likes to time its storylines, we likely won't meet up again with Father Gabriel for another couple of weeks, and possibly not until the midseason finale. And even then, will the show even explain what's affecting him? Will they get Dr. Carson back to the Hilltop for Maggie, or will Eugene's allegiance to Negan be too strong for that?