I'm pretty sure Rick has known about everyone in the group having the dormant virus since Dr. Jenner whispered to Rick. Remember thatthe doc did bloodwork on everyone so he knew. I'm not sure everyone in the world is necessarillyinfected.For example there were dead people in cars on the highway that were not walkers. But maybe the collision caused a trauma to their head and that is why they did not reanimate.I think that maybe anyone who comes in close contact with a walker gets the dormant virus. Maybe airborne, maybe from contact. I think there is only one virus. If it gets into your bloodstream you get the fever and die then become a walker. If it stays out of your blood but you die from another cause, you still become a walker.