Ten-year-old Susannah Jackson is dying of what may be the world's newest and oddest disease - an illness so rare and mysterious that science has not yet come up with an official name for it.
Sitting outside her family's mud hut, near the small town of Lui, Susannah is gripped by a series of brain seizures which force her neck to arch forwards, down, and then up again.
No wonder people in this isolated corner of southern Sudan call it "nodding disease."
The disease is puzzling doctors and scientists
"We have no clue as what is causing this. It's like a detective novel and a murder mystery, because it's fatal," says Dr Mickey Richer, a tropical disease specialist from Unicef.
So far, almost 300 children are known to have caught the disease - all in one small region of the country.
Bizarrely, the seizures normally occur when the sufferers start to eat, or when it is particularly cold.
When Dr Richer asks for a bowl of sorghum to be placed in front of Susannah, the "nodding" begins almost immediately, and stop when she has finished eating.
Curiously, Susannah does not react if she eats unfamiliar food - a chocolate bar for instance.
Susannah's eight-year-old brother, Jacob, is now in Lui's crowded hospital with the same disease - at a more developed stage.
He shows signs of being mentally retarded and physically stunted.
During a particularly fierce seizure he threw himself into a fire, and is now being treated for severe burns to his leg.
Two other children in the same ward have almost identical stories.
An hour's drive from Lui along a rutted track lies the village of Amadi - a silent cluster of huts trapped in a forest of thick green grass.
Amadi is now considered to be the epicentre of the disease - 12% of children here are affected.
Last year, experts from the World Health Organisation did neurological scans on some of the children, enabling them to confirm that this is a specific and unique condition.
About 12% of children in Ramadi are affected
Twelve-year-old Ruben Nicholas stands in the middle of the village, gripped by a sudden seizure.
It looks as though someone is forcing his chin down onto his chest.
"He is nodding three times a day," says his father, Nicholas Lado.
"The disease stops him from growing. His brother died from the same disease, in August."
Dr Richer gets out her stethoscope and starts examining a dozen children. A small boy called Maika Philip stares dully into the distance. He looks about 11, but his mother confirms that he is 18.
So what could be causing this horrific outbreak? If this had happened in western Europe, it seems likely there would be at least some answers by now.
Here, in poverty-stricken southern Sudan, families still do not know if they should be trying to quarantine their children.
"If one child has nodding, we separate them from the other children, because maybe it's through air we don't know," says Reverend Sosthen Amen Lati, whose son is affected.
Some villagers say the disease is a curse, others blame the country's long civil war and suspect that government forces have been dropping chemical weapons on Lui and other rebel-held areas.
A toxicology report commissioned last year by the United Nations comes close to ruling out chemical warfare, and also plays down the likelihood of any link to local diets or food production.
At the same time it cautiously raises a few intriguing theories, including the possibility of infected monkey flesh.
Most victims live near Yeri River in southern Sudan
International relief aid is also mentioned. Some locals have admitted to eating donated seeds, meant for planting not consumption, which are coated with toxic substances.
But Dr Richer's hunch is more straightforward. She notes that the victims are all concentrated near the Yei River.
She also points out that 93% of those surveyed are infected with a parasitic worm which causes Onchocerciasis (also known as river blindness).
The level of infection among children without "nodding" is 63%. The worm is carried by black flies which breed near fast-flowing rivers like the Yei.
Could it turn out to be the killer in Dr Richer's mystery?