Although in this case the relevant factor is that since humans originally evolved in Africa, it had more diseases that co-evolved with humans. Hence why South America, which has a cilmate similar to Africa had an even lower disease burden than Europe. At least until Europeans brought Africans, some of whom were infected with African diseases there.

You might want to look at Africa on a map, because it isn't all tropical.

You seem to be thinking of Africa as a tiny place with uniform geography and climate and biome - it's not.