Just as an aside. As a geology student I can tell you that anything biological underground in Hawaii is likely to be crispy fried due to all the magmatic chambers under the island and the fact that the island isn't all that old. Also the idea of multiple earthquakes is very unlikely as we know from the news casts from season 1 (The two things, Part 3) it is in multiple cities (Detroit, San Diego, New York, Houston, Chicago) across america and internationally. None of other mentioned (save San Diego) cities are near a Fault system, so a tectonic hazard is out of it.
Also the Saul's Antibiotics didn't save the infected person, but what if the are acting to prevent Infection. It is mentioned that they stop the reproduction of bacteria right? So what if they are stopping the initial infection from spreading, acting as an inhibitor and slowing the growth of the zombism. This might lead to Saul gaining a form of immunity, as now he has been infected with a weakened version of the virus (like an inoculation) due to the bacteria being weakened from the Antibiotics in his system.