When you run, flailing and crying and soiling yourself while screaming "JESUS CHRIST I'M COVERED IN BEES," they will chase you for over half a mile.
Africanized bees owe their existence to science. Warwick E. Kerr created them in Brazil during the 1950s by crossing a European bee with an African bee. He wanted a bee that could live in the jungle. He got a bee that swarms by the hundreds of millions, is insanely territorial, mindlessly aggressive, has killed anywhere from a few dozen to a few thousand people. And, can live in the jungle.