BAMAKO, Mali When you begin to go blind from trachoma, the first thing you feel is an eyelash scraping your eye.
As for the horror of leprosy, it often begins with a puzzling rash. Among the first signs of polio are fatigue, vomiting and back pain.
Yet these diseases are on their way out. We in journalism mostly focus on problems, but one of the remarkable changes in the developing world has been the decline of these ancient scourges.
When I first traveled through West Africa, as a student backpacker more than 30 years ago, I was haunted by the beggars disabled by blindness, leprosy and polio. Now I'm on my annual win-a-trip journey with a university student, Erin Luhmann of the University of Wisconsin, and she is encountering a fundamentally improved landscape than the one I saw when I was her age.
Take blindness. It has many causes, but one of the most painful is trachoma, which turns the eyelid inward. The lashes then continuously scrape the cornea.
"My eyes felt as if someone had thrown a handful of sand in them," Nawara Souko, who suffered from trachoma for years, told us. Her husband is dead, and, without sight, she found it difficult to farm or care for her five children. Three died.
Then Nawara received a 15-minute surgery from a public nurse trained by Helen Keller International, an American aid group. Sometimes the surgery, which straightens the eyelid, comes too late to restore vision. In Nawara's case, the operation ended the pain and she could see again.
Erin and I watched trachoma surgeries in a village 100 miles west of the Malian capital of Bamako. Villagers who for years had endured agony one woman compared it to childbirth, except that it goes on for years had their lives transformed.
Yagare Traoré said she had spent years in her hut, unable to farm or care for her 11 children, six of whom died. Then she received the surgery, and, after the bandage was removed, a boy stepped forward to guide her home.
"Get out of my way!" she recalled telling him. "I can see! I can walk by myself!"
The cost of this surgery here in Mali is less than $40 per person, according to Shawn Baker of Helen Keller International. So the next time you hear that humanitarian aid is "money down a rat hole," well, think of Yagare Traoré.
Prevention of trachoma is even cheaper. Train villagers in improving hygiene and distribute antibiotics at a cost of less than $1 per person, and trachoma disappears so that people don't even need surgery.
Then there's polio: Only 223 cases were reported last year, down from 350,000 in 1988. Islamist extremists in Nigeria and Pakistan have murdered vaccination workers, but the disease is still inching toward eradication.
A third triumph is leprosy. It can cause hideous disfigurement, including the loss of fingers, toes, ears and the nose, as well as blindness.
Yet a cheap three-drug therapy cures leprosy easily, and a new blood test simplifies diagnosis. The progress is stunning. In 1985, there were 5.2 million people worldwide with leprosy, and now there are fewer than 200,000.
Unfortunately, not everyone gets treated in time. One of our saddest encounters on this journey with Erin was with a 10-year-old boy named Muhammad Bako who had already lost toes and fingers to leprosy.
"I'm fine," Muhammad told us, but he didn't look it. He walks awkwardly with crutches, and his eyes burn with fear and the unfairness of it all.
Muhammad is being treated at a 57-year-old leprosy hospital in Niger run by SIM, a Christian missionary organization. The hospital receives about one new leprosy case a month, down from more than 500 a quarter-century ago.
The progress goes far beyond these three ailments. The number of children dying worldwide before the age of 5 has plunged from 12 million in 1990 to 6.9 million in 2011.
As the disease burden declines, the economy surges. Africa is now booming economically, and six of the 10 fastest-growing economies in the world are on the continent. Don't think of Africa as a place to pity, but as a place to invest.
Journalists and humanitarians understandably focus on unmet needs, and that can leave the impression that the story of global health is a depressing one of failure. In fact, it's an inspiring story of progress. We need to do more, especially against AIDS, malaria and tuberculosis, but one of the great achievements of humanity in recent decades has been the marginalization of ancient and dreaded diseases.
That's why it's possible for me to travel with Erin in some of the most impoverished countries in the world, and feel a glow of hope.
Reach Nicholas Kristof at Twitter.com/NickKristof.