Three years ago, Jesús Tilano went to a hospital in a thickly forested valley in Colombia with large open lesions on his nose, right arm and left hand. He was diagnosed with leishmaniasis, a parasitic disease that is spread in the bite of a female sand fly and which plagues poor people who work in fields or forests across developing countries.

He was prescribed a drug that required three injections a day for 20 days, each one agonizingly painful. Mr. Tilano, 85, had to make repeated expensive bus trips to town to get them. Then his kidneys started to fail, which is a common side effect of the drug, as are heart failure and liver damage.

“The cure was worse than what I had before,” Mr. Tilano said.

Leishmaniasis is a terrible disease, with terrible treatments that have hardly changed in a century. The drug Mr. Tilano took was first given 70 years ago. All the treatments are some combination of painful, toxic, expensive, or challenging to administer, requiring an inpatient hospital stay or daily visits for a month.

Among the so-called “neglected tropical diseases,” many experts believe leishmaniasis is in a class of its own in terms of the lack of progress, in the 120 years since it was first identified, to help the two million people who contract it each year.

Now, finally, that is starting to change: When Mr. Tilano’s grandson Andrés Tilano, 14, contracted leishmaniasis last year, he was treated in a clinic in Medellín, with an experimental therapy that cured his infection in days.

The treatment he received is one of several being developed by the Program for the Study and Control of Tropical Diseases, known as PECET, a small research institute based at the University of Antioquia in Medellín. In its effort to hunt for new treatments for leishmaniasis, the program has partnered with the Drugs for Neglected Diseases Initiative, or DNDi, a nonprofit research and development organization based in Geneva.