I would say the average Africans immune system is armed to the teeth from exposure to everything. There's also the lower age and higher fitness of the general population as well. Not so many fat, diabetic couch surfers in most of Africa, where in SA its closer to 'western' lifestyles. They also are very in-tune with epidemics and avoiding getting sick, you wont see many of them laughing at the spread of sickness like in the west.