Virologists call it the "hot zone" -- nature's version of a nuclear ground zero, the centre of an onslaught by one of the most deadly biological agents ever known to humankind.

Kailahun, a poor but resourceful trading post like any other in Sierra Leone until a few short months ago, has found itself at the epicentre of the worst-ever outbreak of the feared Ebola virus.

No one gets in and no one leaves the eastern districts of Kailahun and neighbouring Kenema without special government dispensation, as part of an emergency quarantine.

"You cannot mess about here: this virus will kill you. One mistake, one wrong move, and you're dead -- that's it," a senior aid worker in Kailahun tells AFP.

The death toll from an Ebola outbreak that began at the start of the year stands at 1,145 in four afflicted west African countries: Guinea, Sierra Leone, Liberia and Nigeria.

Kailahun, the traditional home of around 30,000 mainly Mende tribespeople, and Kenema account for the lion's share of Sierra Leone's 810 cases and 384 deaths.

Getting there from the capital Freetown involves a drive of seven to 10 hours, depending on the weather and the mood of the police and soldiers at nine checkpoints.

A black-topped highway bordered by palm trees and flat, endless savannah eventually gives way to a treacherous mud track lined by crocodile-infested swampland, dense forests of African teak and swifts darting between the Khaya trees.

Here the security is suddenly less laid-back.

Police bark questions at each traveller, demanding proof of accreditation to go further. At three of the posts, people are made to wash their hands in chlorine and have their temperatures taken.

"We are so sad because our brothers and sisters are dying, so many of them," says Ahamadou, a police guard at a post on the border between Kenema and Kailahun districts.

- Song and prayer -

"We need the world to be aware that we need a vaccine. That is the only thing that is going to stop this."

There is no evidence to suggest that this is true.

Ebola is one of the most deadly and contagious pathogens known to man, and no proven cure or vaccine clinically trialled on humans exists.

A walk through the steamy streets of Kailahun is an unexpectedly uplifting experience, given that almost everyone knows someone who has died of Ebola.

Children play noisily to a soundtrack of fire finches, egrets and emerald cuckoos, chasing old tyres in the iron-rich red mud, while adults throng the streets exchanging money, black-market petrol and laughter.

Ebola -- a haemorrhagic fever that kills most of the people it infects -- is not highly contagious, but can spread easily in crowds where people are exposed to each other's bodily fluids.

A cough might not do it. But a sneeze in the face, contact with infected blood or sweat, or a handshake with someone caring for an ailing, incontinent relative easily could.

Yet people squeeze together at the Friday market and the town, a far cry from the suspicious, frightened capital, comes alive with the song of all-night anti-Ebola prayer sessions.

But the atmosphere changes a short walk uphill at the specialist Ebola treatment centre run by global aid charity Doctors without Borders, known by its French acronym MSF.

Here, there is little joy, nor reason to laugh -- only death and suffering.

The centre runs like a military camp, everyone following procedure to the letter, every patient and health worker divided by location and clothing according to their risk status.

Up to 15,000 litres (3,300 gallons) of water and 2,000 litres of chlorine are used every day to clean hands and clothes, and a huge incineration pit takes care of everything else.

- 'Sometimes they all die' -

Confirmed Ebola patients are fed using an intricate arrangement that ensures the uninfected are never exposed to danger.

Ebola is a disease that allows little time to wallow or grieve, despite the bonds the workers form with their patients.

Patients brought to the centre leave with certificates confirming their recovery, or in heavily disinfected body bags.

There have been survivors -- 52 of them so far, from some 200 confirmed cases -- who are psychologically debriefed and carefully placed back into their communities.

They owe their lives to a hard-pushed rota of expatriate volunteers, hygienists, support staff and local nurses, who work punishing hours to save as many as they can.

Geraldine Begue, a 31-year-old anaesthesia nurse from Luxembourg who quit her job in Switzerland to volunteer at the centre, starts her day at 6:00 am taking blood samples before turning to feeding, hydration, and organising pharmacy stocks and staff.

Depending on when patients come in, she can still be at the centre at midnight, taking patients through triage and admitting them, including entire families.

"Sometimes they all die. Sometimes just the parents recover, and most of the time all parents die and only one child survives," she says.

Most of her patients suffer severe diarrhoea, vomiting and agonising pain as their organs break down, which is eased by morphine and tramadol.

"Ebola is really a messy virus," she says.

It is an emotionally testing environment, and while Begue is disarmingly cheery, she admits that sometimes it can get on top of her.

She and her colleagues work in the certain knowledge that for a good proportion of the patients they treat, it is a question of providing a comfortable death rather than saving a life.

Sia Lavalley, 30, is in the high-risk area. Her husband Tamba died at the centre along with her seven-year-old boy, Tamba Jr.

"I'm feeling the pain. I have pain in my joints, which they are treating," she tells AFP from behind two plastic fences that create a protective buffer zone a few feet wide.

"The worst pain I feel though is whenever I see the other children here running around, sick with Ebola."