So in this last chapter Mike says Tanya and some scratches and blood on her but she in not turned. Even though these could just be brush burns / scrapped from being dragger. They could have also came in contact with the blood from the zombie that was dragging her. And since Mike says shes not turned, just passed out.

Do you think Saul and Tanya somehow have something in them that causes a immunity without knowing it?

Or is it just luck that both of them had open wounds and were close to getting infected blood on them and didn't?