Define Evolution of virulence?

Diseases like cholera emerge as sudden outbreaks, showing marked variation via space and time both in their incidence--the number of individuals infected in a population--and in their virulence--the damage a pathogen does to its victim in the course of implementation its life cycle by replicating and infecting other hosts. Epidemiologists have long studied how frequency of viral, bacterial, and metazoan parasites can fluctuate in response to modifications in infectiousness and transmission which are in turn driven by social, economic, and medical trends. More currently, they have begun to appreciate that virulence is as well dynamic, evolving fast in response to similar factors that drive pathogen numbers.