Saul and Tanya immune hypothesis.

1) The pick line treatment.
2) Blood exposure + pick line treatment.
3) Genetic immunity.
4) The Yet To Be Discussed built-up immunity because Saul lived near Ingelwood in Compton and could have been exposed to what ever underground 'zombie stuff' in slow, steady doses. A problem with this is that those closer should have a better tolerance. Any thoughts or suggestions that would make #4 worthwhile?