Think the WHO doctors specifically mentioned it should be a bacterial strain vice viral.
Yeah, but as you noted bacterial meningitis is not something you want to have, spreads quickly and is quickly lethal. That's why I went with viral meningitis (again there was mention of a "meningitis strain" over the ending montage) as it's more manageable but does fly in the face of the WHO doctors not thinking a viral strain would be sufficient. Though I'd argue they were speculating as much as Gerry was since we don't know what everyone else had that made them camouflaged.
The guy in S. Korea we can only speculate on. He either simply had a tweaked leg, arthritis, MS or literally anything. All we know on him is that his leg was a symptom of whatever disease he had that made him undesirable to the zombies.
The old man in Israel is pure speculation even more so he could have anything that made him undesirable.
The kid in Israel, due to him being skinny and bald, likely had some form of cancer.
I do think viruses are unlikely since we all have viruses in us, every virus we've ever gotten sick with in fact. Just that our bodies have learned to cope with them to keep them in check. Though maybe an active virus would be enough? One you're currently, actively, sick with? But, again, the problem with that is how much is "lethal enough"? The "common" seasonal flu kills 10s of 1000s of people a year, a cold can be deadly to a person with a weak immune system.
We know that physical liability isn't enough since Segen's amputated hand didn't help her any. (This means the officer in Korea and the old man had something beyond physical injury.)
Bacteria strikes me as unlikely as well since a walking corpse would be a bacteria farm.
We know whatever Gerry infected himself with had a cure as we see him getting it at the end. (A vaccine for a virus wouldn't work after infection.)
The whole idea just doesn't hold up very well after much though. I like the idea of it as it was "different enough" to be interesting it just doesn't hold up as it's not well explained. Perhaps in the sequel?
Two things of note on it though, Gerry's suggestion seemed to be for it to be used for those fighting back but in the ending montage we see children and civilians given it. This may further suggest that whatever they settled on wasn't "that bad." Hell maybe just having it and built the immunity to it was enough? Also during the ending montage were told by a newscaster that the camouflage had been "largely" successful. The word use there suggests it's not 100% effective.
Anyway, nitpicking aside I did like the movie as, well, I like Brad Pitt in almost anything. I do, however, strain to think of where Pitt's "oral fettish" came into play in this movie other than him drinking the [Pepsi] at the end.