Environmental history has demonstrated that human agents do not drive history strictly on their own powers, but rather work in interaction with the natural environment and under its constraints. Human actors have certainly altered the natural environment, but the environment has also presented human beings with surprises, some of them as unpleasant as they were unpredictable. When human beings began to cultivate crops and domesticate animals, for example, they concentrated plants in fields and animals in herds. Little did they realize that they were creating ideal habitats for pathogens and pests that preyed on their crops and flocks. Even less did they realize that some of these pathogens and pests could thrive on human as well as animal hosts.