The fact that this keeps having to be explained just proves how important it is that this game receives some story telling and lore. Without any context, most (if not all) players who play this game think, "I've just been bitten/scratched by a zombie and I have an infection debuff. I must be infected with the same virus that turned these people into zombies." It's common sense and a completely logical conclusion to come to.
Now, of course the game is still in EA and I know TFP have said a "story" is coming. I just hope that when that comes, a little detail a given to explaining how you are possibly immune to the zombie virus and that getting injured by them doesn't mean you are going to turn when that counter reaches 100%. Otherwise players will continue to ask the same questions.
I always love posts like this. Like, what is the expected outcome? We know the alpha is going to be released soon, but TFP aren't going to rush things just because it might disappoint a few players. There will be a group of players who will be disappointed either way. Releasing a week or two sooner than you would like isn't going to change that.