As it does today, malaria played a huge role in the past--a role unlike that of other diseases, and arguably larger. When Europeans brought smallpox and influenza to the Americas, they set off epidemics: sudden outbursts that shot through Indian towns and villages, then faded. Malaria, by contrast, became endemic, an ever-present, debilitating presence in the landscape. Socially speaking, malaria--along with another mosquito-borne disease, yellow fever--turned the Americas upside down. Before these maladies arrived, the most thickly inhabited terrain north of Mexico was what is now the southeastern United States, and the wet forests of Mesoamerica and Amazonia held millions of people. After malaria and yellow fever, these previously salubrious areas became inhospitable. Their former inhabitants fled to safer lands; Europeans who moved into the emptied real estate often did not survive a year.