This can be explained as follows. Although zoonotic disease emergence is not entirely a tropical phenomenon, it is mostly associated with tropical developing regions undergoing the most rapid population growth and ecological changes. Prior to the post-WWII economic era, most regional ecosystems in the tropics consisted of relatively scattered human settlements, and only a few large cities (>500,000) (24) . These were separated by large expanses of cropland and pastureland and relatively undisturbed forest. Since then, in what has been the most rapid period of large scale ecological transformation in human history, the pattern has essentially reversed (25) . The once scattered settlements and few large cities have coalesced into expansive megacities and surrounding periurban settlements with only remnant patches of undisturbed forest remaining in a sea of cropland, scrub, and ecologically degraded lands. Dacca, Bangladesh, which grew from a population size of 200,000 to 13 million from 1970 to 2003, is one of many examples.