You got bit around the start of the outbreak, and nothing happened, the bite mark eventually healing. In fear of what they would say or do, you told nobody. It seemed unrealistic anyway. If you told someone, anyone you were "immune," they'd probably laugh in your face.

Here you are on a supply run, just you and Daryl. You were killing a walker when it bit you, not horribly painful but noticable. Luckily, Daryl didn't notice yet, but he heard your cry of pain.

"Hey, you good over there?!"