I am very interested to see where the story is going with Saul. I don't think the antibiotics had anything to do with it, but rather his genetics. It could explain a little about Tanya. Perhaps she knows she is immune (may have been scratched by a Zed at one point and realized she didn't turn). Maybe she is afraid the physical at Ft. Irwin will show that she has had an encounter with a zombie and she is afraid they will kill her. I'm thinking she and Saul have the cure in their blood. But I don't want to go too far out on a limb with what little information we have. It's just something to stew over.

Regarding the cracks in the ground, I immediately started thinking that it could have been a bacteria laden gas that escaped from deep in the Earth. If tectonic activity released it all around the world, that could explain the seemingly simultaneous appearance of zombism internationally.