Maybe Jenner told Rick that everyone is a carrier but Rick didn't really understand what that meant.
Yeah Jenner didn't have time to explain everything. He probably just said, "you're all infected." How do you interpret that? We're all infected and therefore immune? We're all infected and therefore going to turn into zombies at any moment? Or maybe Jenner is just nuts or lying - how can you be infected and still apparently healthy and unaffected?
Now, telling everyone that they're all infected may destroy any hope the group had for something that resembles a normal future
Or he can tell everyone the good news that they don't have to worry about being nipped by a zombie because they're already resistant enough to the virus that it hasn't turned them into zombies already. Zombies can't kill with just a bite, they can only kill through "normal" means such as massive blood loss.
That's actually an improvement
in their circumstances. They just need to be careful to always shoot anyone who dies through the head, but that's simple enough to adjust to. All the adults should be carrying loaded weapons at all times and this would be a good time to go hit up a sporting goods store for crossbows and start training everyone.
The one downer would be if the virus will eventually turn them into zombies while still living, without warning, because there's no defense against that. And I think that's the case, given the way Shane was depicted in the last episode. But they have no reason to know that yet, and that shoe won't drop till sometime next season at the earliest.
From there the show becomes a show about how this immunity gets turned into a cure, and ditches the will humanity survive Angle.
The ratings are strong and the show needs a plot arc that will keep it going for years. I'm having a grand old time right now watching the survival angle. They've got a new game to play for next season, with the impact of Dale's death inspiring them not to lose their humanity. That could form most of the plotline for next season, especially if they run into other survivors who will challenge that ethos.
They could easily save the "we're all going to turn into zombies while alive" angle till S3 and segue into Rick as the hope for the cure at the end of that season, or in S4. It's all about pacing. But simply sticking to the survival angle is going to run out of steam before the ratings do. Compared with other shows, this one has a very straightforward and simple premise, which is good, keeps things focused and powerful, but it's just too simple for a show that AMC probably wants to run for years - and that, well managed, can run for years.