The Europeans forced labored the Africans, moving them into close villages, and making them work for hours on end. Because of this small distance, disease spread very quick.
The Africans were catching the diseases quick, and when the Europeans left, the Africans were devastated by it. The Europeans left without giving any help to the Africans, therefore leaving them to die.
Many of the Africans died, and in some attempts to clear up disease they burned the houses and corpses, losing their homes and suffering even more. At the end of the day, it was a genocide. Huge amounts of Africans died, killed by disease, or European ways.