One drug can treat three deadly and neglected infections - Chagas disease, leishmaniasis and sleeping sickness - animal studies show.
It has been described as a "new hope" for tackling the parasitic infections which affect millions of people in the poorest parts of the world.
The discovery, reported in the journal Nature, was made by testing three million compounds.
The new drug is now entering safety tests before human trials.
The three diseases are all caused by similar parasites, leading scientists to believe one therapy might be useful against the trio.
Sleeping sickness is caused by the Trypanosoma brucei parasite, which is spread by the bite of the tsetse fly. The disease is officially known as Human African trypanosomiasis, but takes its more common name from the coma that results when the parasite penetrates the brain. It is found in sub-Saharan Africa.