When the phrase “zombie apocalypse” first entered popular culture in the late 20th Century, it functioned primarily as a chilling metaphor for societies overwhelmed by social decay and the erosion of human agency. As severe drug epidemics later swept through parts of Western society, from America’s heroin crisis of the 1970s through the 1990s to the crack cocaine wave of the 1980s, the metaphor acquired a more visceral realism.