Focusing on colonial Algeria ca. 1890 to 1940, this paper explores what Muslim intellectuals and ordinary people learned about microbes and how they responded to bacteriological medicine. Many Algerians feared invisible spirits (jinn) and sought the healing powers of saints and exorcists. Was it then permitted for Muslims to use French treatments and follow Pasteurian rules of hygiene? Specialists in Islamic law, other intellectuals, and unlettered villagers showed a persistent concern with these and other questions in the wake of colonial conquest and violence in Algeria, as novel techniques, therapeutics, and forms of epistemic authority were introduced, and new visions of religious orthodoxy and national revival were formulated. This paper examines jinn theories and germ theories across a range of genres and formats—including newspaper advertisements, poetry, cartoons, and radio broadcasts—to show that Islamic tradition and law were integral to the emerging science and culture of microbes in early twentieth-century Algeria. While Islamic reformists sought to displace jinn theories of illness, other Algerian intellectuals and colonial officials found it convenient to explain germs in terms of jinn. Both French and Muslim elite men sought to combine religious law with hygienic advice to advance competing hegemonic projects targeted at the Muslim family, thereby attempting to displace women’s jinn-based practices.