The Black Plague is believed to be caused by rats and the bad sanitation and the water they drank.
You might be talking about the Out-of-Africa Theory or early
migration of early modern human species out of Africa. According to this
theory, our modern human ancestors left Africa about 200,000 years ago. They
left in two waves; one died out or retreated while the second populated the
world. At that time, Africa experienced very long droughts, so early humans
have to migrate to other greener areas. Therefore, Africa at that time is not
forested and humans did not move out onto savannah regions.