“I came in on Sunday morning, checked the tissue culture under the microscope in the hood and took out fluids from all the eggs, and found out that the virus had grown.”He was thrilled. Some contained chicken eggs, which are usually used for growing ordinary flu viruses; others contained tissue culture made from canine kidney cells.”Four of us were working flat out,” says Mr Douglas, 55, a research associate who was in the laboratory all last weekend More people were being hospitalised in Turkey as he worked. This was then analysed.The rest of the sample was shared out between dishes in which the virus might grow. His sisters Fatima and Hulya also died.”I knew the boy offered our best chance of isolating and growing the virus,” says Mr Douglas, who diluted the sample, and from part of it extracted the RNA, the genetic material that is the virus equivalent of DNA.

He had died from the pneumonia-like symptoms of avian flu which have hospitalised 78 other people across Turkey. It is thought that Mehmet and his siblings had been playing with the severed heads of infected birds. Mr Douglas and his colleagues work with their hands and arms thrust through a portal into gloves, or look into a microscope that extends out of the cabinet as a hood.The best of the samples was a swab or slice from the lungs of Mehmet Kocyigit, a 14-year-old boy who had lived on a farm thousands of miles away in the village of Dogubayazit, in the mountains close to the border with Iran. One by one they were placed in an airtight chamber, then passed into a main cabinet. All waste leaving the lab is sterilised.The Turkish samples came in plastic tubes, protected by bubble wrap, in a box containing carbon dioxide to keep them at the right temperature.

Black polythene is taped up at the windows to prevent sunlight from getting in. This is to stop unwelcome particles from ruining experiments, but it is also to stop viruses with which the scientists work from escaping into the atmosphere.The air pressure inside the building is lower than outside, so that if an accident happens air will be sucked inside rather than out. He unlocked the door and entered the laboratory, the air in which passes through a complicated series of filters, each sterilised with formaldehyde. Only four people have permission to enter the laboratory: the security manager and three scientists.Before starting work, Alan Douglas showered and left his clothes in a decontamination room, then dressed in royal blue overalls very like a surgeon would wear in an operating theatre, with a white laboratory coat.

From the outside, the lab looks like the boiler house on a 1960s polytechnic campus, with pipes and ducts sprouting from the walls. The main block is a tall building with a presence unsettling enough to have been used as an asylum in Batman Begins.The samples were driven down the hill overlooking the parklands and big houses of Totteridge – “where the millionaires live”, as one member of staff on government wages puts it – to Containment Four, the laboratory with the highest level of security. As it swept through red and white security barriers the passenger, a woman from the Ministry of Health in Turkey, may just have felt she had seen the institute before. They were driven to Mill Hill from the airport in a car with Turkish diplomatic plates. Few outsiders ever see the lab, but last week The Independent on Sunday was given unprecedented access.The head of the centre, Dr Alan Hay, cut short a holiday when he heard the news of an outbreak in the Van region of Turkey. The other scientists cancelled days off as samples from the dead boy and his sister were flown to London, a week ago last Thursday. The centre is one of four labs that receive samples from new victims of bird flu – but as the others are in America, Japan and Australia, it is London that will test the European outbreaks first.

The protein spikes on its surface had changed and it was now more attracted to people than birds. This was not the nightmare scenario, insisted Sir John Skehel, but it was “one step along the way”.He is director of the National Institute for Medical Research at Mill Hill, which contains the World Influenza Centre on its 50-acre site. Two days ago the World Health Organisation announced that the H5N1 virus found in the body of a 14-year-old boy from a village in Turkey had mutated. They knew people were dying, but that their results would tell doctors which drugs to use against the virus.They were also looking for evidence of the big change that the world dreads. The faster they can do so, the more lives will be saved.”We do have a great responsibility here,” says their boss, Sir John Skehel. “Flu research requires that you accept that.”Alan Douglas is one of the scientists, a 54-year-old research associate who last week became the first person to isolate and grow the strain that had killed three children in a village in Turkey. He and his colleagues at the World Influenza Centre worked flat out on their samples for days.