So there we were, Paul and I, “enjoying” a 50-cent-per-person “breakfast” at a “restaurant” here in the town of Koundara in northern Guinea when we first came face to face with blindness on this trip.

Image Nicholas D. Kristof Credit... Fred R. Conrad/The New York Times

A man named Amadou Bailo shuffled toward us, holding one end of a stick as his daughter held the other and walked ahead of him. In wealthy countries, the blind have seeing-eye dogs; in poor countries, the blind have seeing-eye children. The girl, Mariama, who thought she might be about 9 years old, has never been able to attend school because she spends her days guiding her father. Her older brother was the father’s guide before that, so he never went to school either.

“His blindness has kept two of his children from going to school,” noted Shawn Baker of Helen Keller International, an aid group that works on vision and nutrition issues. Mr. Baker met us, after we had crossed over from neighboring Guinea-Bissau, at such a sleepy border post that we had to wake up a border guard.

Mr. Bailo had lost his sight from an excruciating affliction called river blindness, which is caused by baby worms that infest the body and destroy the optic nerve. River blindness was once endemic in much of West Africa and seemed almost hopeless. Yet today, in one of the great triumphs of humanitarian workers, it is under control and perhaps close to being conquered.

Credit goes to former President Jimmy Carter for helping to lead the fight against the disease, to a number of aid groups and to Merck, which donated the medicines to kill the baby worms. Mr. Bailo will never recover his vision, but these days, virtually no one in West Africa is going blind from the disease.