One hundred years ago today, on the morning of the 31 October 1914, a 20-year-old private ventured out into firing line of the First World War for the first time.

We know from frontline reports that he and his platoon had just left their trench when they were "found" by the German artillery.

The explosions sparked chaos and confusion as everyone dived for cover. The young soldier was separated from his comrades and became tangled in barbed wire.

As he struggled to free himself, three shells rained down on him, missing him by only a few feet. Witnesses said it was sheer miracle that he survived.

But when the young man was admitted to hospital a few days later, it was clear to the medics that his close brush with death had left a mark on him the like of which they had not seen before.

History hasn’t remembered the young private’s name. Today we know him only as "Case 1" from a seminal report published early in 1915 by a Cambridge professor and army doctor called Dr Charles Myers.

It detailed the first documented cases of what Myers came to describe as "shell shock".

More than 80,000 members of the British Army had been diagnosed with the disorder by the time the First World War came to an end, including the famous war poets Siegfried Sassoon and Wilfred Owen.

It was the first time that the authorities had ever been confronted with mental health trauma on such a scale.

Today most of them would have almost certainly been diagnosed as suffering from forms of post-traumatic stress disorder (PTSD).

But when Myers was writing, the science of psychological injury was still in its infancy. And the Army didn’t know how to respond.

At first the breakdown of men under the intense pressures of conflict was seen more as a question of character. In the worst cases, patients were accused of cowardice or even desertion. There are appalling accounts of young men who desperately needed treatment for their condition but who were instead court martialled and sent to face the firing squad.

Soon it became clear however that the condition could not be written off as cowardice.

Myers came across countless soldiers mentally wrecked by what they had gone through. They included an officer haunted by memories of going out into "No Man’s Land" to look for a missing soldier only to find his friend’s body blown to pieces.

By the end of the war, Myers and his colleagues had developed pioneering techniques to treat their patients. A real but gradual change began in the way they were viewed by society.

When the Second World War came decades later, the psychological toll of combat was much better recognised. Army doctors were trained to recognise and treat the symptoms. The importance of rest and recovery away from the frontline was much better understood.

This progress is all part of a journey that we are still on a century later.

Today the long-term effects of combat stress are much more deeply studied than ever before. But the problem has not gone away.

Roughly one in every 25 British troops who served in Iraq and Afghanistan are at risk of developing PTSD, with the proportion much higher among soldiers in direct combat roles.

The mental health charity Combat Stress – founded a year after the conclusion of the First World War – currently has a caseload of more than 5,400 veterans across the UK.

Last year they reported a 57 per cent increase in veterans from Afghanistan being referred to them – a number that is expected to rise as the last UK forces depart Camp Bastion.

The latest figures show that on average it takes 13 years after a veteran is discharged before they seek treatment with Combat Stress, many of them having tried and failed to access the help they need through the NHS.

It feels like a modern echo of challenges that our soldiers faced a hundred years ago. It underlines why we cannot afford an ounce of complacency.

This week alone experts from the Royal College of Psychiatrists and others have raised concerns that the government is failing to honour the military covenant.

A cross-party report by MPs has separately highlighted how many veterans suffering from PTSD risk falling victim to alcohol abuse, domestic violence, even entering the criminal justice system.

A century on since doctors first examined that young private – "Case 1" – there is clearly still much more we need to do to help those we ask to put themselves in danger for our country.

Mental health stigmas are diminishing, but they have not yet disappeared. This moment from history shows it’s time we prioritised the mental health not only of our veterans, but of everyone.

Dan Jarvis is the MP for Barnsley Central, a former Major in The Parachute Regiment, and Labour’s lead on the First World War centenary