I'll bet a box of donuts he told Rick that they're all infected and they'll zombify when they die, bitten or not. Remember? When they asked about the blood tests Jenner didn't say they were okay. He said, "No surprises." Either that, or that he watched his buddy try to rape his wife on the security cam.
The virus was probably dumped in the water supply or something. That's how it spread and the first walkers appeared. There had to be a first zombie who wasn't bitten. It makes sense.