Immunity != ResistanceA human with a strong immune system is RESISTANT to most diseases. He destroys pathogens in his system.Geralt is not resistant. He is GENETICALLY IMMUNE. Which means pathogens that enter his system cannot trigger activity within themselves as his body environment does not provide the trigger. Conversely, his body doesn't see these elements as threats, and does not take active measures to eliminate them.Inactivity in pathogens does not mean their death. They can remain in hibernation till they are passed on to a host in whose body their activity can be triggered. This is pure speculation, however, as we do not know exactly what kind of immunity the witcher's mushroom juice game them.And I also can't believe we are discussing this in such a serious note.