Answer:
Disease
Step-by-step explanation:
It was hard for the Europeans to colonize Africa because they could not stay in their land because of tropical diseases like Malaria. Later on Quinine a crystalline substance which is present in cinchona bark was discovered and helped Europeans be able to stay inland and then they colonized Africa!
(This is the only information that repeated and was the most accurate about why the colonization of Africa was slowed down. Have a great day!)