In The Walking Dead, what did Dr. Jenner tell Rick?
It is revealed in Season 2, Episode #13 "Beside the Dying Fire". Jenner tells Rick that
No matter how you die, you always come back as a walker - in other words, everyone is already infected. This comes after other survivors ask how Randall turned without being bitten and after we see Shane come back without being bitten. However he did have a scratch on his stomach.
That's not true otherwise Rick would have mentioned that to Shane in the episode where they found the two guards with no bite marks on them. Its not explained that you turn even if not bitten until season three.