Professor Callahan teaches courses in human disease, animal disease, veterinary immunology, construction of self in the arts and sciences, and creative science writing. He has published poetry and creative nonfiction in numerous literary and scientific journals. His books include River Odyssey: A Story of the Colorado Plateau; Faith, Madness, and Spontaneous Human Combustion: What Immunology Can Teach Us about Self-Perception; Infection: The Uninvited Universe; and Between XX and XY: Intersexuality and the Myth of Two Sexes. Two of these books were finalists for the Colorado Book Award in general nonfiction. He is currently under contract with University Press of Colorado for two additional books: Basic Veterinary Immunology and Lousy Sex: A Marriage of Art and Science.  He investigates interactions between the human immune and nervous systems, humans and their microbiomes, and the biological basis of sex. His work has been featured on or in, among others, National Geographic Television, ABC Evening News, Los Angeles Times, Salon.com, Chicago Tribune, National Public Radio, Minnesota Public Radio, Talk Radio Europe—Spain, Ms. Magazine, New Scientist, TV Ontario, and other venues