I'm going to guess he told Rick that everyone is already infected and that the virus takes full effect when a body 'dies' or hits a necrotic state. The only thing against this is that the people on the highway who were dead did not reanimate. However, there could be a loophole that either they died before the infection spread to them or that they just haven't reanimated... yet.
Although we'll know what it was he whispered to Rick by the end of season 2. It's been confirmed.
I don't think humans, at least in the world of TWD, can carry around the virus without succumbing, at the most, days after infection. The only zombie-infested world I've read about with dormant zombie infections was in the book Feed.
I assume rates of infection depended on the circumstances. People who were bit but got away died later of the infection, not the wound. However, someone like Amy, who bled out from the massive bite in her neck, died of the wound but the virus eventually "traveled" to her brain, resulting in her not reanimating for several hours. Here is where my theory goes a bit off because I'm not sure viruses can survive in the stagnating blood of a dead person. However, this is a zombie virus we're talking about.
Also, I know humans are always covered in bacteria, germs and dormant viruses, and it's actually good for our immune system to have exposure to the weak stuff to prepare for stronger strains (which is the idea behind vaccines). However, rapidly mutating viruses change their genetic material at such a rate to make antibodies obsolete to their new form. This is why there is no "cure for the common cold", because the commonness of it ensures several and rapidly mutating strains.
All this slippery medical knowledge that you can totally correct me on leads me to my ultimate point: I don't think the humans in TWD had any sort of immunity toward the virus, considering the rate of infection and the few weeks it took for the world to go to s**t. And I assume something this infectious would know how to survive, something assured greatly by mutating and perhaps its speed. Clearly the virus works fast so an immune system wouldn't have a chance to eliminate it. Therefore, I don't think Jenner told Rick about everyone being infected. Frankly, I think the producers of the show completely forgot to add that to their meandering plot.