In 1859, Charles Darwin published On the Origin of Species by Means of Natural Selection, a literary work that not only provided a framework for the theory of evolution, “descent with modification” by natural selection, but also explained how the cumulative impact of natural selection affected an organism and its environment. Darwin, however, neglected to mention the way in which infectious diseases represented a fundamental selective force in natural selection (Lederberg, 1999). Since animals first walked the earth, they have had to coexist with microscopic organisms, including bacteria, viruses, fungi, and parasites. These microorganisms not only shared the environment with animals, but interacted with them, causing disease and death. Humans were no exception, humans have had to adapt to these microbes on a genetic, as well as social, level, implementing environmental changes to survive. (Inhorn & Brown, 1990). In this article I aim to explore how infectious diseases have impacted human adaptations by demonstrating a coevolutionary relationship between humans and the microorganisms that humanity has sought to evade. Infectious diseases can be defined as an invasion caused by organisms ranging from microscopic viruses to bacteria to complex parasites (Inhorn & Brown, 1990). According to Inhorn and Brown, it can be argued that infectious diseases have been the primary agents and driving force of natural selection over the last 500 years, selecting for those humans who were most susceptible to a pathogen and selecting for those individuals who were most susceptible to a pathogen. resistant to a pathogen. Inhorn and Brown based their premise on evolution at the microscopic level. The organism... at the center of the paper......ase, a biological arms race that is still ongoing today. ReferencesAndré, J.-B., Gupta, S., Frank, S., & Tibayrenc , M. (2004). Evolution and immunology of infectious diseases: what's new? An electronic debate. Infection, genetics and evolution, 4, 69-75. Barrett, R., Kuzawa, C. W., McDade, T., & Armelagos, G. J. (1998). Emerging and re-emerging infectious diseases: the third epidemiological transition. Annual Review of Anthropology, 27, 247-277. Inhorn, M. C., & Brown, P. J. (1990). Anthropology of infectious diseases. Annual Review of Anthropology, 19, 89-117. Lederberg, J. (1999). Anecdotal, historical, and critical commentary on genetics. (J. F. Crow and W. F. Dove, eds.) Genetics, 153, 1-3. Porter, R. S., & Kaplan, J. L. (Eds.). (2011). The Merck Manual of Diagnosis and Therapy (19th ed.). Whitehouse Station, New Jersey: Merck Sharp & Dohme Corporation.