The introduction to Microbiology and Horror Cinema addresses four types of pathogens—funguses, bacteria, parasites, and viruses—and the ways that they overcode the formal designs, narrative codes, and representational conventions of such films as The Exorcist (William Friedkin, 1973), The Wicker Man (Robin Hardy, 1973), The Skeleton Key (Iain Softley, 2005), Annabelle (John R. Leonetti, 2014), and Crawl (Alexandre Aja, 2019). Much as COVID-19 and other microorganisms infect us, damage our organs, and reproduce inside of us invisibly, so too do the real anxieties that these films touch upon remain difficult to see, unless we take a more epidemiological approach to them. Microbiology and Horror Cinema theorizes such an approach, as it teases out the true sources of fear that these films stir up in us, namely those that have to do with contracting, spreading, and suffering from some form of debilitating, disfiguring, and life-threatening disease.