When Makoy Samuel Yibi Logora was growing up in a village in southern Sudan, no one there knew what caused Guinea worm. But they certainly understood its effects. The skin swells and becomes infected as a thin white parasitic worm takes several weeks to emerge slowly, agonisingly, through a huge blister.

The only thing that can be done medically at this stage is to resort to the ancient technique for easing the worm's laborious exit: winding the worm - which often grows to 3ft long - around a stick and coaxing it out, bit by bit. Some scholars believe this procedure is the origin of the ancient Greek symbol of healing, the rod of Asclepius enveloped by a serpent, which many medical bodies around the world still use as part of their logo.