What if there are two groups, those immune and those not. Only thing is that these groups are relatively evenly split, like 3/4 of the population is not immune and 1/4 is. Now the infection is spreading like wildfire among those not immune. But it soon becomes clear that theres something else going on. After being infected you say... become much like a L4D common infected (except more dangerous... lets make them all hunters actually). But after the infected kill someone who is immune the transformation is soon reversed. The infected person returns to normal and is now immune to the infection. They will NEVER become a zombie again, and on top of this, they are fully aware of everything they did and was done to them when they were a zombie, but they were completely out of their own control.
Now don't go putting words in my mouth to make this simpler, i said that the zombie has to KILL to return to normal. I don't want any creative solutions like they only have to drink the blood of someone who is immune, not without making the zombies more of a threat so that curing them is not only difficult, but also requires some sort of sacrifice on the part of those who are immune.
