shaved his head in the bathroom earlier in the season? It could have been the lighting of my tv or even my imagination, but I saw open sores on the back of his head/neck. I'm not speculating on the sores itself being related to the virus, but it could be an easy way for it to enter the body.
I really like the airborne virus theories that have sprang up so far, but just in case it isn't airborne, this gives a little evidence of a droplet infection. Then the virus could enter the body through a wound and lay dormant until the victim dies. And to my knowledge, I think everyone on the show has had an accident of some kind and contracted the virus that way. From Carl's gaping gunshot wound to maybe Carol scraping her knees raw when Daryl tackled her to the ground to keep her from going to zombie!Sophia.