Before Europeans took over the Africans were spread out so they didn't have much disease nor were they in close proximity to each other. They also worked in places that had small amount of people. This prevented disease from spreading. Also,They didn't have a government then Europe crated a government.
When Europe came they created cities and a government that favored them. Example: English, forced labor in mines, Africans couldn't enter certain parts of the country without permission. Europe would take people from 100 sq. miles and make them all work in mines and live in a much denser place. Since it was denser the disease spread much faster and easier.
The only reason Europeans came to Africa because they needed resources fuel the Industrial Revolution. The europeans aren't going to work themselves so they got africans to do it for them. Europeans had guns germs and steel so even if it was 100 vs 3. The 3 would win because they had guns germs and steel so that helped them stay in control. So that the tribes didn't gang up on them, they created conflict between the tribes so that they wouldn't gang up on them. When the Europeans left they took all the resources from Africa and left Africa without leaving a leader. Then the Military General became the ruler which wasn't appropriate and that caused political corruption.