Abstract

Evolution is a central concept in biology. Indeed, when one considers definitions for life, perhaps the simplest and most elegant definition for life is that life consists of things that evolve. As medicine is a subfield of biology, evolution is central. Microbes are essential for all vertebrate life, for functions including digestion, nutrition, and defense. A microbe does not “want” to cause disease or not cause disease. All life on earth has been selected for billions of years to reproduce successfully, and this is all that matters from an evolutionary standpoint. If pathogenic traits provide an evolutionary advantage in a given situation, they will be selected for. If they provide a disadvantage, they will be selected against.

In comparative medicine, we often lack information regarding both host and pathogen species. When information is lacking on a given species, the best model to use is typically the closest relative from which data is available. This requires knowledge of species relationships. There are a number of important selective pressures impacting microbes in a vertebrate host, which lead to differing degrees of host/microbe relationships and differing host fidelity. Significant factors impacting host/microbe relationships will be discussed, and examples given. Understanding pathogen ecology and evolution may identify agents of concern before significant outbreaks occur, enabling management strategies for risk mitigation to be proactive rather than reactive.