"Are you feeling OK?" surgeon Georges Bwelle asks the prisoner lying on a makeshift bed.
He picks up his scissors and forceps and returns to operating on a small hernia.
The music playing in the room next door in the rural jail grows louder, and the doctor sings along.
During the week, Bwelle, 49, is a top doctor specialising in intestinal surgery at the main hospital in Yaounde, the capital of Cameroon.
At the weekend, though, he takes to the road.
He heads a lively team of volunteers who cram into a minibus and head out into remote areas -- a tiny mobile clinic that provides basic healthcare to those in need.
Today, his NGO, called ASCOVIME, has gone to the prison at Nkongsamba, a town lying in green hills about 350 kilometres (215 miles) northwest of Yaounde.
The team is welcomed by the prison administrator and an NGO called Agriculture for Africa, which is hosting the trip.
Today, almost 500 prisoners and their families will be examined.
A clean room is provided near the cells.
"It's perfect," says Bwelle.
Immediately, a chain of volunteers forms to bring in the equipment, transforming the room into a small field hospital, complete with departments for general medicine, ophthalmology, dentistry and even minor surgery.
The inmate, 35, emerges from the operating room, his hernia now fixed.
"Thank God I have been freed from this ailment -- the doctors looked after me," he says, clearly moved.
Bwelle's drive to help others can be traced to his childhood.
Born into a family of modest means, the young Bwelle saw his father's health deteriorate because of the lack of access to a specialist doctor following a road accident.
After his studies, Bwelle began to travel throughout his country.
"With the little money I had, I bought medicines and treated three or four people, then 10, then 100," he told AFP.
Little by little, a team of doctors with a panoply of skills gathered around him.
In 2008 he set up ASCOVIME, an acronym in French meaning Association of Skills for a Better Life.
Today, the NGO carries out about 40 missions a year, provides medical consultations to 40,000 people, carries out around 1,400 operations and gives school equipment to 20,000 children.
In Cameroon, a central African country of about 25 million people ruled for more than 38 years by Paul Biya, almost four people in 10 live below the poverty line and life expectancy is around 60.
Most of the time, ASCOVIME visits rural areas where health care is scarce and difficult to access, including two English-speaking regions plagued by separatist conflict, and the far north, affected by jihadist incursions by the Boko Haram group.
"In every village, there is at least one medical hut run by a nurse, often a Cameroonian government employee," he explains.
But the needs are almost limitless -- "the main problem is poverty and lack of equipment."
The most frequent medical complaints are malaria, joint pain and hernias -- diseases linked to working the land, says Bwelle.
In a summer camp atmosphere, the bus -- its sides emblazoned with the English words "Local initiative to reduce people suffering" -- traces its route to the rhythm of traditional Cameroonian music.
Dressed in shorts and a navy blue sweatshirt, Bwelle, who also teaches at the University of Yaounde, adds his voice and claps his hands.
He grabs his young proteges by the shoulder, tells jokes, and beams.
"It's a bit silly, but I love making people smile. And for me, that means treating those who need it," Bwelle says.
Emmanuella Mounjid, a sixth-year medical student, is one of the weekend volunteers.
"I am learning a lot about medicine. Specialist doctors come with us and answer our questions," she says.
"But the experience is the richest at the human level. There is nothing more beautiful than receiving a smile."