More than 200 NHS staff are being put through their final training in the UK before being deployed to Afghanistan.
Most are from the Territorial Army's 256 Field Hospital, which draws volunteers from London and south-east England.
For the next two days they will be based in a giant warehouse outside York, that has been adapted to create the conditions they will encounter at Camp Bastion in Helmand.
The huge hangar used to be a storage depot for armoured vehicles, but for all practical purposes it is now a field hospital, meticulously designed to reflect the layout at the camp.
It is teeming with doctors, nurses, surgeons, paramedics and other healthcare staff who are leaving behind their NHS duties for a while to help with the war effort in Afghanistan.
The sound of casualties coming in is signalled on the PA system, against a steady din of helicopters blasting from speakers dotted along the ceiling.
In a neighbouring block, dozens of actors are being prepared for medical attention.
Some of them are amputees, some do not speak English, many of them are elaborately made up with horrendous injuries - each presenting a different challenge based on recent cases at Camp Bastion.
Lt Col Jan Pilgrim, chief instructor at the training centre, said the set-up was very authentic.
"What we have at the moment is an A&E department that's receiving casualties, a theatre department with two tables in that is priming up ready to treat some of those casualties, and then the rest of the hospital - intensive care, lab, X-ray, set out as you would have normally in an NHS department."
Staff urgently assess the first patient of the exercise. He has bloody wounds on his arm and foot - a mortar shell casualty, they are told.
All the equipment they need is on hand, with trainers supplying further information on symptoms to help them reach decisions.
But this is not a test of clinical competence. It is about creating a team that can work well together under extraordinary pressure.
Field hospital work is likely to be relentless and challenging. And some here, like Clare Hayes-Bradley, will head out into the battlefield to pick up casualties.
An intensive care specialist based in Stevenage, Hertfordshire, she is used to working on an air ambulance in the UK, but she thinks her role in Afghanistan will be tougher still.
"Often with a road traffic accident the injuries are inside," she said.
"It's internal bleeding you can't see, and we have to rely a lot on scanning and imaging to see the injuries.
"With the soldiers they do have internal injuries and the explosions can send shrapnel and little fragments all over their bodies, but also if someone's lost both their legs it's very graphic and very visual and it think that's going to be a huge difference".
A lot of the NHS staff on this exercise have never been deployed overseas before - though preparations for this have been under way for 18 months. For them, there is a mixture of excitement and apprehension over what is to come.
In her civilian role Capt Marion Creagh is a nurse at the Royal Surrey Hospital in Guildford, Surrey. She is confident this training will stand her in good stead for her time in intensive care at Camp Bastion.
"It will be challenging but I've always embraced any challenges that I've had in the past. I've looked forward to this deployment, going out, doing the best I can for the soldiers on the ground."
And how will her NHS colleagues in Guildford cope while she disappears to the other side of the world for three months? A surgical matron at the trust, Julie Burgess, says they will manage.
"I think because we're quite a large post-anaesthetic care unit we can absorb the loss of Marion for those three months. On busy times we'll call on other resources into the unit to take up the work that she would be doing at that time."
And after Christmas, when it is all over, staff from 256 Field Hospital will have to re-adapt to life - and work - back in the UK.
Many expect the intensity of their deployment will help them bring new skills, perhaps into new career opportunities in the NHS.
Their commander, Col Peter Gilbert, says they will be so absorbed in their work, the time will race by.
"We're going to be so busy there that almost as soon as we arrive and start getting into it, it will almost be time to be preparing to hand over to the next group.
"We'll be away for Christmas, but of course most of us will have Christmas in January when we get back, so it's not too much of a problem."