Paralysis. Stuttering. The 'shakes'. Inability to stand or walk. Temporary blindness or deafness.

When strange symptoms like these began appearing in men at Casualty Clearing Stations in 1915, a debate began in army and medical circles as to what it was, what had caused it and what could be done to cure it. But the numbers were never large.

Then in July 1916 with the start of the Somme battle the incidence of shell shock rocketed. The high command of the British army began to panic. An increasingly large number of men seemed to have simply lost the will to fight. As entire battalions had to be withdrawn from the front, commanders and military doctors desperately tried to come up with explanations as to what was going wrong. 'Shell shock' - what we would now refer to as battle trauma - was sweeping the Western Front.

By the beginning of August 1916, nearly 200,000 British soldiers had been killed or wounded during the first month of fighting along the Somme. Another 300,000 would be lost before the battle was over. But the army always said it could not calculate the exact number of those suffering from shell shock. Re-assessing the official casualty figures, Taylor Downing for the first time comes up with an accurate estimate of the total numbers who were taken out of action by psychological wounds. It is a shocking figure.

Taylor Downing's revelatory new book follows units and individuals from signing up to the Pals Battalions of 1914, through to the horrors of their experiences on the Somme which led to the shell shock that, unrelated to weakness or cowardice, left the men unable to continue fighting. He shines a light on the official - and brutal - response to the epidemic, even against those officers and doctors who looked on it sympathetically. It was, they believed, a form of hysteria. It was contagious. And it had to be stopped.

Breakdown brings an entirely new perspective to bear on one of the iconic battles of the First World War.