This evolutionary perspective suggests that diseases spread by per- sonal contact should generally be less virulent than those conveyed by insects or other vectors. Do the facts fit this expectation? They do indeed. Among Paul Ewald’s important discoveries is the truth of this generalization and its importance for public health. He has shown that diseases from vector-borne pathogens tend to be more severe than those spread by personal contact and that mosquito- borne infections are generally mild in the mosquito and severe in ver- tebrate hosts. This is to be expected because any harm to the
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mosquito would make it less likely to bite another vertebrate. For gastrointestinal pathogens, the death rate is lower for direct, as com- pared to waterborne, transmission, as long as really sick hosts can effectively contaminate the water supply. As pure water became the norm in the United States early in this century, the deadly Shigella dysenteriae was displaced by the less virulent Shigella flexneri. As water was purified in South Asia during the middle of the century, the lethal form of cholera was steadily displaced by a more benign form, and the transition took place earliest at the places where water was first purified.