Asymptomatic carriers can introduce the organism into new populations.
Over time, most people acquire antibodies to the etiologic agent. This may account for the fact that most disease occurs more often in persons under the age of 20. Sporadic cases and epidemics occur throughout the world, but much of Sub-Saharan Africa also has endemic disease. In the past, outbreaks have occurred in Brazil, Kenya, Tanzania, and India. More recently, Ethiopia and Saudi Arabia have experienced outbreaks.