Microbes have enormous potential mechanisms of genetic diversity - they evolve, adapt, and emerge in response to nonmicrobial and even non-biologic changes in the physical and social environment.

The development of disease is dependent on the extent of exposure to an agent, the strength or virulence of the agent, the host’s susceptbility, and the environmental conditions. Considering the epidemiological triangle - host, environment, and agent - what is not clear is how these three change over time, often in response to changes in another side of the triangle.