Chapter 29-1- Beyond Our Walls Part 1

[Witch_Doctor]

Theory no 1: Saul and Tanya are immune. Saul had more then just one opportunity to get infected (the hybrid at the tank-station, being shot while fending off Tommy); and Tanya was bitten by a slow-turner.
Theory no 2: Saul and Tanya were quick enough to use the right, hard medication to prevent the infection to spread in their body.

About Lizzy: No, I think Lizzy is not immune. Spoiler Aler: But 'something' may become immune regarding her; how much of the show have you listened to yet?

Another Spoiler Alert: Hope ... (Just a mad theory made by me ...) This, again, depends of how far you have listened into the show yet ...

All the best!
Liam

Ever since you brought up this idea I keep thinking that perhaps that what ever caused her blindness may have prevented some sort of Z contagion from reaching the brain.