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Virulence, the degree at which a pathogen causes fatality in its host, exists as a by-product of the trade-offs between elevated ability to replicate, and infect new hosts (Ewald, 1991). High virulence increases chance of host death, but can enhance the pathogen’s likelihood to spread. The water-borne nature of Vibrio cholerae allows the bacterium to transmit at a faster rate via contaminated water sources. This adaptive trait confers higher accessibility to hosts and decreased reproductive cost (Ewald, 1991). Thus these types of pathogens typically evolve higher levels of virulence than those reliant on direct transmission (Ewald, 1983)

Ewald, P.W. 1991. Transmission Modes and the Evolution of Virulence: With Special Reference to Cholera, Influenza, and AIDS. Human Nature 2(1):1-30. http://link.springer.com/article/10.1007/BF02692179/

Ewald, P.W. 1983. Host-Parasite Relations, Vectors, and the Evolution of Disease Severity. Annual Review of Ecology and Systematics. 14:465-485.