As hunter/gatherer populations evolved into farming communities, the risk of disease increased. Environmental changes, such as land-clearings in forests, irrigation canals, and rice cultivation, created patches of stagnant water where mosquitos could breed. And agricultural lifestyles encouraged people to live in greater proximity, making it easy to mosquitos to transfer disease-causing microbes from person to person.

The population explosion in the last thousand years has accelerated these trends. More crop lands were created to feed people, and denser communities—which were more economically and culturally viable—were formed.