Lankien - In the middle of a piece of fenced-off ground stands a large tree. Its trunk is gnarled and ancient, its crown round and full and the lowest branches hang close to the ground. Even the African sun can scarcely penetrate its leafy canopy. The sand around its trunk has been neatly swept; there is no litter on the ground.

But this is not just a tree: it is the only hospital for miles around.

In the shadow of its branches patients lie on food-aid sacks stitched together. Their shoes and sandals lie neatly arranged next to them. They serve as a bedside table - a drinking beaker in the right shoe and a bag of pills in the left shoe. The doctor, the nurse and the healthcare workers from Medécins Sans Frontières (MSF) walk from one patient to the other. They have a couple of low wooden benches with them to sit on, a folding table and a big tin chest full of medicines, hypodermic needles, sterile water, cotton wool and bandages.

Many of the patients under the tree are suffering from kala azar, a tropical disease that occurs frequently along the upper reaches of the Nile in the south of Sudan. At the beginning of the nineties, an epidemic cost the lives of a hundred thousand people.

"You catch it almost as easily as malaria from the bites of sandflies and it wastes you as if you have HIV/AIDS," says Dr Erwin de Vries. "In a few months time you will look like a skeleton and you will be dead."

At the present moment, approximately 100 kala azar patients are being treated under the tree.

"For a doctor this is a rewarding disease," says De Vries. "It can usually be cured within three weeks."

Seriously ill patients are put on a drip. The plastic bags of fluid can be easily hung from the branches. The other patients get two injections a day in a mud hut next to the tree, which hurt so terribly that they alternately receive one in each buttock and one in each thigh.

Nyadat Wie sits in the shade of the tree with her six-year-old daughter and her baby of a few months old. A year and a half ago her brother and his son died from kala azar. Now her daughter has the disease too and will soon get her first injection.

"Life has been better since there has been peace," she says. "We have a little to eat. The soldiers stay calm and we can go to the clinic if necessary."

Even a simple hospital under a tree can make a big difference. In November, the team from Medécins Sans Frontières (MSF) visited 22 surrounding villages - on foot because there are no roads - to ask how many people had been ill and died in recent months. For an area in which war had raged for decades, the answers gave reason to be hopeful - the health situation was less bad than in other African crisis areas. The majority of people who felt ill had been looked after in the hospital in Lankien or in one of the five MSF health posts in the surrounding area.

MSF has been working in Lankien only since June 2005. A year previous to that, the team was evacuated because of the violence. The hospital was plundered and the buildings were partly destroyed. The local population presented MSF with four white bulls - the traditional way in which a reconciliation is marked in Upper Nile - and promised that no one will any longer carry weapons in the neighbourhood. Six new huts once again stand in a semicircle around the tree, where the patients sleep and can shelter from the rain.

But Lankien is still one of the few places in the Upper Nile region where people with kala azar and other diseases can get medical help. Few doctors work in the south of Sudan; outside the cities, MSF and a couple of other aid organisations are the only ones to provide healthcare. It often takes days to walk to a health post, which means that people hesitate for a long time before embarking on the journey and can sometimes no longer be treated.

Standing under the tree, Erwin de Vries says, "We can only guess how many people die because they live too far away from here."