It's not possible to turn this off, as otherwise a game will go on for ever. Unlike in Necroa (where zombies decay) there is no mechanism for apes to die off naturally, so the game would keep going on and on

Yes, but there are ways to evolve in an animal species and reinfect humans... It's not that important, but it would expand the ability for people to make wildly different scenarios if they were able to change this. I could probably do a work around by setting the entire planet as infected from the start though for my purposes.