Odilon Redon's dark-spirited charcoals and lithographs of the last quarter of the nineteenth century responded to developments in science, including the Pasteurian revolution. Rather than celebrating the progressive potential of science, Redon's noirs engaged national anxieties that attended scientific advances. His position was close to the Decadents of the 1880s who dwelled on themes of illness and decay. While the artist's original biographer André Mellerio referred to the artist's fascination with Pasteur and his microbial world, where in a single drop of water “there arises the spectacle of giants of a gripping horror and a frightful, predatory nature,” the topic has virtually disappeared from Redon literature. By recovering the scientific/medical issues that stimulated Redon's creation, this article restores the subjects of pathogenic organisms and contagious illness to their rightful place in Redon's oeuvre. Further, it provides the reader with a discussion of the cultural context in which Redon's interest was generated.