Evolution is defined as a gradual process in which organisms become better adapted to their environment through gradual changes that occur from generation to generation. Throughout the history of life, the human species has changed to better adapt to the environment. All changes are ultimately the result of mutations that occur at the genetic level. Pathogens such as bacteria or viruses that live inside our cells have had a great influence on our evolution (Parks, Panelli & Weinstein, 2003). Pathogens have influenced our evolution in two main ways, which I will focus on: antibiotic resistance and virulence. Virulence has also changed in terms of how it reproduces and develops a higher fitness rate. In addition to pathogens, the human body has evolved to develop processes in which cells are better protected (Parks, Panelli & Weinstein, 2003). These processes are referred to as defense mechanisms or the immune system. Due to constant changes in human health and medicine, evolution has been selective to provide the human species with a higher level of fitness. At the cellular level, genes are made up of DNA, which is the genetic blueprint for growth and development. When DNA is replicated and undergoes cell division, errors, known as mutations, can occur. After multiple generations of offspring are produced, the mutations will eventually change enough to show a new distinguishable trait such as a disease or particular disorder (Ovchinnikov, Rubin & Swergold, 2002). If the same genes are passed on and carry the traits of a particular disorder, the host's fitness will be reduced, due to the impact of a particular disease. An example would be cystic fibrosis, which develops... halfway through the paper... ca, 88(10), 4270-4274.Madigan, M., & Martinko, J. (2006). Brock biology of microorganisms. (11th ed.). New York, NY.McDade, T. (2005). The ecologies of human immune function. Annual Review of Anthropology, 34, 495-521. Ovchinnikov, I., Rubin, A., & Swergold, G. (2002). Tracing the lines of human evolution. Proceedings of the National Academy of Sciences of the United States of America, 99(16), 10522-10527. Parkes, M., Panelli, R., & Weinstein, P. (2003). Converging paradigms for environmental health theory and practice. Environmental Health Perspectives, 111(5), 669-675. Futuyma, D. (2009). Evolution. (2 ed., pp. 421-424). Sunderland, MA: Sinauer Associates, Inc. Willard, H., Angrist, M., & Ginsburg, G. (2005). Genomic medicine: genetic variation and its impact on the future of healthcare. Philosophical transactions: biological sciences, 360(1460), 1543-1550.