Sierra Leone's dense tropical rainforest and swampy environment were considered impenetrable; it was also host to the tsetse fly, which carried a disease fatal to horses and the zebu cattle used by the Mande people. This environment protected its people from conquest by the Mande and other African empires, and limited the influence of the Mali Empire. Islam was introduced by Susu traders, merchants and migrants from the north and east, becoming widely adopted in the 18th century.