Stage 2 : point of no return.
The victim becomes aware that they're furrifying, but can't go back. It struggles, trying to get out of the infection, but at this point it's too late.
Stage 2 can last longer if the victim is resilient, but can't be reversed. At this point, It is better to let the victim be infected than trying to save it, as it could trigger the opposite effect.
It is important to note (for ethics purposes) that at this point, the victim is still human.