Long-term relationships between pathogens and their hosts are often characterized in military terms; the two are locked in an “evolutionary arms race,” in which, to survive, each must prevent the other from getting too far ahead. When an entirely new pathogen shows up, it’s like bringing a gun to a knife fight. Never having encountered the fungus (or virus or bacterium) before, the new host has no defenses against it. Such “novel interactions,” as they’re called, can be spectacularly deadly.
