Why is Sub-Saharan Africa seemingly so disease-ridden compared to the rest of the world? Is it just because humans have lived there longer than anywhere else, so a lot more diseases are endemic to humans? Is it just an artifact, perhaps, of European perception?
No, it's definitely disease-ridden. It's the mosquitos, and other parasitic forms of life. There are worms in all the water, it's just a death-trap.
It wasn't as bad before. The natives had strategies for dealing with it, but the imperial powers thought Black people were retards and made them stop all their "inefficient" practices, which caused disease to spread everywhere horrendously.
I'll give an example: Bunyoro used to fight sleeping sickness by completely burning the entirely of the grasslands every year (killed off the tse-tse fly) and massive game hunts, which killed off primary disease carriers. The British stopped both, causing the population of Bunyoro to plummet, and it continued to shrink from the time of conquest until the 1950s.