World distribution of onchocerciasis.
From the WHO website: http://www.who.int/pbd/blindness/onchocerciasis/en/
More than 17.7 million people worldwide are infected with onchocerciasis. Visual impairments occur in 500,000 of these people, including 270,00 cases of blindness. Ninety-nine per cent of all cases of onchocerciasis occur in Africa. In some communities in West Africa, 50% of the men over age 40 have been blinded by onchocerciasis. [h, i, Markell, and m (quoting k and l)]
Onchocerciasis is found in rural, often poor, populations living near fast-flowing rivers, rapids, and other turbulent waters where the blackfly vector breeds. Studies show a correlation between the proximity of villages to the water and the prevalence and intensity of onchocercal infection occurring in those villages, giving rise to the designations first-, second-, and third-line villages. [a, Markell]
While the women of most villages draw water from village wells, the men more often fish and farm near the fast-flowing rivers. Thus, males are more frequently infected, more severely infected, and more often blinded by onchocerciasis than females. [a, h]
The prevalence of infection is nearly zero at birth, since onchocerciasis can, but seldom does, undergo intrauterine transmission, and increases with age to a high plateau later in life, reaching 100% in hyperendemic areas. Similarly, intensity of infection increases with age to a plateau between ages 30 and 50 years. [a, Markell]