How the pharmacist survived for so long
Here is a potential explanation for how the pharmacist survived for so long after being bitten:
Similar to how people actually start to believe a lie after telling it for so long, the pharmacist began to believe he truly was immune after pretending to be for so long.
This belief, in turn, caused a psychosomatic effect on his body which boosted his immune system, and helped to stave off the infection for far longer than most people would be capable of.