Trinitrotoluene, or TNT, was first used in artillery shells by the German Army in 1902. Soon after the First World War started in 1914, a rain of these devices was falling on the hapless men on each side of the front. It was a level of violence and horror far beyond the cavalry charges of earlier wars. Very quickly, soldiers began emerging with bizarre symptoms; they shuddered and gibbered or became unable to speak at all. Many observers were struck by the apparent capacity of these blasts to kill and maim without leaving any visible trace. The British journalist Ellis Ashmead-Bartlett famously described the sight of seven Turks at Gallipoli in 1915, sitting together with their rifles across their knees: “One man has his arm across the neck of his friend and a smile on his face as if they had been cracking a joke when death overwhelmed them. All now have the appearance of being merely asleep; for of the several I can only see one who shows any outward injury.”

For those who survived a blast and suffered the mysterious symptoms, soldiers quickly coined their own phrase: shell shock. One period lyric went like this:

Perhaps you’re broke and paralyzed

Perhaps your memory goes

But it’s only just called shell shock

For you’ve nothing there that shows.

One British doctor, Frederick Mott, believed the shock was caused by a physical wound and proposed dissecting the brains of men who suffered from it. He even had some prescient hunches about the mechanism of blast’s effects: the compression wave, the concussion and the toxic gases. In a paper published in The Lancet in February 1916, he posited a “physical or chemical change and a break in the links of the chain of neurons which subserve a particular function.” Mott might not have seen anything abnormal in the soldiers’ brains, even if he had examined them under a microscope; neuropathology was still in its infancy. But his prophetic intuitions made him something of a hero to Perl.

Mott’s views were soon eclipsed by those of other doctors who saw shell shock more as a matter of emotional trauma. This was partly a function of the intellectual climate; Freud and other early psychologists had recently begun sketching provocative new ideas about how the mind responds to stress. Soldiers suffering from shell shock were often described as possessing “a neuropathic tendency or inheritance” or even a lack of manly vigor and patriotic spirit. Many shell-shock victims were derided as shirkers; some were even sentenced to death by firing squad after fleeing the field in a state of mental confusion.

This consensus held sway for decades, even as the terminology shifted, settling in 1980 on “post-traumatic stress disorder,” a coinage tailored to the unique social and emotional strain of returning veterans of the war in Vietnam. No one doubted that blasts had powerful and mysterious effects on the body, and starting in 1951, the U.S. government established the Blast Overpressure Program to observe the effects of large explosions, including atomic bombs, on living tissue. One of my uncles recalls standing in the Nevada desert as an Army private in 1955, taking photographs of a nuclear blast amid a weird landscape of test objects: cars, houses and mannequins in Chinese and Soviet military uniforms. At the time, scientists believed blasts would mainly affect air pockets in the body like the lungs, the digestive system and the ears. Few asked what it would mean for the body’s most complex and vulnerable organ.

Only after yet another European war broke out did scientists begin looking again at blast’s effects on the brain. When the Balkans collapsed into fratricidal violence in the early 1990s, Ibolja Cernak, a small, tenacious woman who grew up in the countryside of what is now Serbia, was working as a doctor and researcher at a military hospital in Belgrade. She soon began seeing large numbers of soldiers with blast trauma, usually from mortars and artillery fire, a common feature of that war. As in World War I, the men often suffered from striking mental impairments but few visible wounds. Cernak, whose colleagues call her Ibi, has an appealing blend of briskness and warmth, along with a clinician’s conviction that you must listen to your patients. It is easy to imagine her running around the battlefields of Bosnia and Serbia, collecting blood samples from soldiers. That is what she did for several years, at no small risk to her life, for a study cataloging the neurological effects of blast on 1,300 recruits. “The blast covers the entire body,” she told me. “It has a squeezing effect. Ask soldiers what they felt: The first thing they say is that their ears were popped out, they were gasping for air, like some huge fist is squeezing them. The entire body is involved in that interaction.”