Average temperatures today are only marginally higher than they were then. This is the most annoying of all the examples. Until relatively recently, the people of Rwanda lived in isolated homesteads on the steep mountain slopes. There was little malaria. But one of the factors in the terrible turmoil in Africa has been a population explosion.
Arable land is now scarce.
People have started cultivating the valley bottoms, replacing papyrus swamps, where malaria mosquitoes are uncommon, with rice. The rice fields are surrounded by villages. That's where most of the new transmission is occurring. Also, people are traveling much more, carrying the parasites with them. There's also resistance to anti-malarial drugs.