Furthermore, due to its relatively low person-to-person transmission rate, it can only spread throughout the population in the presence of one or more high-risk transmission channels, which are thought to have been absent in Africa prior to the 20th century.
Ademais, debido á súa relativamente baixa taxa de transmisión de persoa a persoa, pode só espallarse na poboación en presenza dun ou varios canais de transmisión de alto risco, que se cre non se daban en África antes do século XX.