Define Historical example of virulence?

The "classical dogma" of epidemiologists was that pathogens all the time evolve in the direction of lower virulence, eventually becoming harmless symbionts. Evolutionary models have led epidemiologists to abandon this easy assumption, replacing it with a much richer view that incorporates the effects of within- host competition, tradeoffs among transmission and virulence, and coevolution between pathogen offenses and host defenses. The tradeoff theory, that focuses on the pathogen's trade off among maximizing its replication and preserving its host's viability, has strongly suggested that public health experts can manage the virulence also the incidence of disease by taking actions that lower transmission rates and shift transmission to less damaging modes, such as from water-borne to direct contact. Easy mathematical models have been necessary in formulating and understanding the conclusions of the tradeoff theory.