The land was left emptied of its cattle and its people, enabling the colonial powers Germany and Britain to take over Tanzania and Kenya with little effort. With greatly reduced grazing, grassland turned rapidly to bush. The closely cropped grass sward was replaced in a few years by woody grassland and thornbush, ideal habitat for tsetse flies. Wild mammal populations increased rapidly, accompanied by the tsetse fly. Highland regions of east Africa which had been free of tsetse fly were colonised by the pest, accompanied by sleeping sickness, until then unknown in the area. Millions of people died of the disease in the early 20th century.