I looked and hadn't seen anybody mention this idea, so forgive me someone has and I missed it. I'm not sure if I buy in to the theory of Saul and Tanya having some genetic immunity to whatever is causing the zombie outbreak, but if they do I had a thought as to possibly why. Saul and Tanya are some of the few African American characters on the show, and the Sickle Cell trait, which is hereditary, is present in 1 in 500 African Americans. Generally, Sickle Cell is a bad thing, but it is actually provides protection against the parasite that causes malaria. Perhaps it could also afford some protection against to whatever is causing the zombie outbreak as well?