Complex sentence: They were virulently malarial until suppression with DDT beginning in the 1950s and 1960s enabled intensive settlement that displaced or disenfranchised aboriginal peoples, cleared forests and modified local environments in ways that may threaten biodiversity and contribute to increasingly severe flooding downstream in India and Bangladesh.
Split into simpler sentences: They were virulently malarial until suppression with DDT beginning in the 1950s and 1960s. Settlers also cleared forests and changed the local environment in ways that may threaten biodiversity and contribute to increasingly severe flooding downstream in India and Bangladesh.