Actually, the book says that there was only a small chance (it didn't specify the exact percentage) that the virus could mutate and become contagious to humans. So it's not just like it would only affect a small percentage of the population. It would be more of an "all-or-nothing" thing. Once it became contagious (however small the chance of that happening might be), it could hypothetically affect everybody. Human race go bye-bye.
So, yeah, I think I might have to agree with Ax on that one. That wasn't an acceptable risk.
That's one thing I hate about that book. That changes in the middle. It starts off "Oh the Yeerks that are affected might die in their hosts' heads and then swell up or something and cause the host to die," which, by itself, isn't that big of a deal. The hosts, at that point, may very well accept death over infestation, and, like we've said, there probably weren't altogether too many human hosts to start off with.
THEN when Arbat is going to dump the virus into the Yeerk Pool (which wouldn't affect the hosts since they're NOT IN THEM), it's all "Oh waah wait it might mutate and affect humans like we said earlier remember except I said something different but now I change my mind and forgot to edit the earlier thing Ax wait why don't u lyke me anymore
"
To go back to how many humans are controllers, though, I think the ambiguity can be blamed solely on the chapter one recaps that start every book.
This may be a new post, and if it is, my apologies, but the thing about those recaps is that annoying as they were, you always had to read them. Why? a) Because usually, you'd get Erek popping up out of no where or some other plot impetus that got the story rolling and b) Very, very rarely, you would get new information.
Sometimes there are thousands of human Controllers.
Sometimes there are tens of thousands.
Sometimes there are "who knows how many humans the Yeerks have infested?"
Sometimes, someone you know might be a Controller.
Sometimes, it's for certain that someone you know is a Controller.
Sometimes, we humans are totally outnumbered.
And sometimes, there's only 17,000 Yeerks on deck to infest the entirety of the human race.
I mean, part of it is that the Animorphs themselves don't know what the **** is going on, but part of it is also that the authors could never quite seem to figure out just how big the Yeerk threat is.
