Dr. Vivien Sil Mabouang, the head of health services in a lush, green district in the center of Cameroon, drives the mud village roads with newfound worry about the health threat from the rivers and streams that thread through the land.

Black flies breed in the fast-flowing water, and when they bite people, they can pass on larval worms that mature beneath the skin. The adult worms can live for 15 years, producing millions of immature worms that move through the human tissue. If the immature worms die in the skin, they cause intense itching, and if they die in the eye, they can cause blindness — so the infection, formally called onchocerciasis, is better known as river blindness.

Onchocerciasis is painful and debilitating, but with crucial funding from the United States, Dr. Sil Mabouang and his colleagues had been close to reaching their goal of wiping it out in their region and having the district officially declared free of the disease.

Then, the Trump administration drastically scaled back foreign aid, and funding for the program to eliminate onchocerciasis was cut off.

Onchocerciasis is one of 21 afflictions, most of them treatable and preventable, that the World Health Organization classifies as neglected tropical diseases. Together, they affect more than a billion people, but because many are among the poorest people in the least developed places, these diseases have historically received little funding, research or attention.

While they are rarely fatal, these diseases exact a huge toll in human suffering, including pain, disfigurement and disabilities such as blindness. They are sometimes called “biblical” because they have plagued humans for so long that they are mentioned in ancient texts.