Right now, only 15 percent of the world's rainforests are still intact. Once pristine rain forest in Borneo, the land on the left has been clear-cut for a palm oil plantation. Everytime people cut into forests, they create "edges," where people come into contact with animals — and their viruses. Tropical regions also have a greater diversity of viruses. By 2050, more than half of the world's population is projected to live in the tropics and subtropics.