Sixty-nine years ago last week, a slender woman named Tomiko Shoji was struck and sent aloft by a bright white light. She’d just arrived at her secretarial job, at a tobacco factory, and was standing by the door when the flash occurred; the light’s source had a nickname, Little Boy, but it meant nothing to her at the time. She flew backward under the crushing force of the office door, passed out, and awoke with shards of glass in her head and an expanse of bodies around her—some dead, some alive but dazed, and many more, she soon found, floating “like charcoal” in nearby rivers. The nineteen-year-old climbed up and out of the shell of her younger self; she had survived the U.S. atomic bombing of Hiroshima. Nearly seven decades later, Keni Sabath, Shoji’s youngest granddaughter, started to wonder: Had the bombing’s aftermath reshaped not just the psyche of her bachan (grandmother) but also, in ways both culturally and historically particular, her own?

In recent years, a public-health hypothesis has emerged that one of the world’s most poorly understood pandemics isn’t a conventional virus—like H1N1, say, or some hemorrhagic fever. This hypothesis suggests that untended wartime trauma can move vertically and horizontally through individuals and families, morphing across years, decades, or even centuries. Sabath began considering the prospect as early as high school, after certain overpowering symptoms emerged on a family visit to Hiroshima when she was six. It was Sabath who had arranged for me to visit her bachan at her aunt’s home in Hilliard, Ohio, where Shoji agreed to share her first full account of the bombing and the family mysteries that followed.

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On my way to Hilliard, I carried my copy of John Hersey’s “Hiroshima.” (My 1989 edition bears the cover endorsement “Everyone able to read should read it,” and I agree.) Its text first appeared as an entire issue of this magazine, on the one-year anniversary of the bombing, and followed the fates of six civilians in the aftermath. Even now, on a more distant anniversary, Hersey’s granular rendering gives an urgency to these stories: of a young clerk, Shoji’s age, who found herself crushed beneath a pile of books; of a Methodist pastor who charged his way back into the city to help, passing victims whose eyebrows had been singed off and women with the flower patterns of kimonos burned into their skin.

As I entered Shoji’s home, on a quiet cul-de-sac, she swept my hand into hers and pressed her cool forehead against mine by way of welcome. Her eldest daughter, Minori, gave me a pair of slippers to wear inside; as the three of us shuffled into the kitchen, where fresh berries and tea cakes awaited, we paused to examine photographs of the Reverend Kiyoshi Tanimoto, the same pastor who weaves his way through Hersey’s narrative, and who also, apparently, stood at the center of Shoji’s. She first came across him preaching in an open-air bazaar in Hiroshima not long after the bombing; he gave her a piece of paper with information about his church, and she soon converted to Christianity. (He later baptized her grandson, Isao, who served as my translator well into the early evening.) Some of the first words Shoji spoke to me in Japanese were about the Reverend: “He would say, ‘Tomiko, why don’t we go all over the world together and tell them of our experiences with the bomb?’ ”

Tanimoto made a second career out of his own suggestion; on the fortieth anniversary of the bombing, Hersey wrote a follow-up story for the magazine, “Hiroshima: The Aftermath,” in which he described the pastor’s extensive U.S. speaking tour to promote peace. But Shoji wasn’t ready to speak freely at the time. This past July, the last surviving crew member of the Enola Gay, the plane that dropped Little Boy, died in Stone Mountain, Georgia, having given many interviews. By then, Shoji had made up her mind that, in her eighty-eighth year, she would share her own account of what happened on the other side of the bomb. So we began right there, with the flash.

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“Radiation! Heat! The wind from the bomb!” When Shoji began to describe her recollections from August 6, 1945, she took on a staccato pattern of speech, gesticulating rapidly. A tiny woman with pixie-gray hair and a sweet, flushed face, she slapped her small hands together and pummelled her head with pinched fingers, as if to imitate flying debris from the blast. At one point, she pretended to fling burned skin across the room like zucchini peels. Then she closed her eyes and went into a deep repose, resurfacing with a sudden phrase: “I’m scared to meet people,” she said, speaking in the present tense of her teen-age self, who might also be herself at eighty-eight. “Something could just blow up. I’ve seen it before.”

On the morning of the bombing—it was 8:15, the start of the workday—Shoji recalls briefly losing consciousness at the Bureau of Tobacco. “When I got up, I ran down to the first floor, down to the bomb shelter,” she said. “All over was smoke; the entire city was covered with smoke. I saw people coming across the bridge just completely black—covered by blood, coming towards us. … The whole city was a sea of fire. And then, at night, it rained black rain.”

Collecting herself, she began walking with colleagues across the city’s many bridges, toward the sea. She caught a train in Hiroshima’s west hoping to find her sister, to no avail; en route were whole trolley cars that had been blown off their tracks, filled with singed corpses. After spending the night, she returned home to find a note from the same sister, which read, “You can find me at the school.” The two stayed in the school turned shelter for some time thereafter, living in a true dystopia. “There were tens of thousands of flies from the dead bodies,” she recalled. “Our greetings to each other became: are you having diarrhea?”

For all those who perished in the bombing, many more survived, day by day. Only later would some, like Shoji, come to discover that the most devastating aftereffects were like ghosts: coming and going on a whim, wreaking forms of havoc often incomprehensible to outsiders and, sometimes, even to those who suffered it.

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I’d always assumed, in ignorance, that to survive the atomic bomb—to be ahibakusha, or “explosion-affected person”—was to have conferred upon you a certain esteem or deference, not unlike that afforded to the bearer of a Purple Heart. Shoji’s family wasted no time correcting me. To be a hibakusha, they explained, was not an honorific but a source of shame, a secret to be closely held. Even grandchildren have often feared telling romantic partners of their grandparents’ experience, worried that their genetic material would be perceived as spoiled goods.

Eventually, Shoji’s family planned for her to enter an arranged marriage with a prominent policeman in Taiwan, where she relocated in her early twenties. They kept her hibakusha status hushed, and refused to allow the two to talk before the ceremony, so as to better seal the secret. “My hands were shaking, holding my bouquet,” Shoji recalled. When her husband learned the news afterward, he spiralled into a rage that never lifted. For the rest of the marriage, Shoji’s daughter Minori said, “He felt he’d been cheated.”

The next several decades brought a parade of physical ailments that were easily traceable to the bomb: Shoji’s eyes and ears gave up early; her insides felt perpetually cold; her teeth fell out, requiring dentures in her forties. But perhaps most debilitating were the psychological symptoms that she didn’t think she could attribute to the radiation. “For thirty or forty years, I was so afraid of thunder and lightening,” she told me, as one of many examples. “It would just crush me. I just lost control.” Raising four daughters was a challenge of another scale. “Nobody understood me; I was like a beggar,” she said, recalling that when her children were young she faced almost daily bouts of overwhelming panic. At night, in dreams, she shouted, “The Earth—the Earth is going to fall!” “At the time, I didn’t know what was affecting me so badly,” Shoji said. “I couldn’t talk about it. Even before I opened my mouth, I would collapse with fear.”

Minori chimed in, gently stroking her mother’s shoulder: “When we would go into her bedroom in the morning, we would see her get so angry—she would throw things. When we were young, I never saw her laugh—she was quiet, and weak.” Back then, neither Shoji nor her children spoke openly about this behavior as tied to the bomb. Remarkably, Shoji says that the idea didn’t come easily to her. She was unfamiliar with the concept of post-traumatic stress disorder, or shell shock, or its classic presentation (nightmares, flashbacks, hypervigilance); these traits seemed unrelated to her experience. “Every year I have these crazy episodes—my family is so good to me, but I have these outbursts, these moments when I lose control,” she said. Years ago, she insists, it all seemed completely inexplicable.

Still, somewhere within her, she began to trace a clear line between her inner state and the events of her nineteenth year. “After I go married, the family would yell at me, and even when I’m beaten, I can’t respond, and I don’t know why. But deep inside, I remember, oh, that’s what it is: the bomb, the aftereffects of the bomb. It’s worse than the day of the bomb.”

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Shoji’s granddaughter Keni Sabath grew up in Hawaii and Texas, the child of a New Jersey-born Navy JAG officer and a fashionable Taiwan-born language tutor. Like her older sister Zena, Keni often spent her days with her bachan, who lived in their home for years before joining Keni’s aunt Minori in Ohio. In the summers, the family would travel back to Japan. “I first became aware of my grandmother’s experience in a very disturbing way, when I was six years old,” Keni Sabath told me after my Ohio visit. “I went to the Peace Park in Hiroshima with my grandmother and my mom. We walked by the river and my mom would translate, ‘This river here was turned into a blood river, and people would jump into it and their skin would burn off.’ ” The family proceeded to the local memorial museum, where life-size wax statues depicted local children fleeing the bombing site, their skin melting and their clothing singed. “The children were my height!” Sabath said. “It was so hard for me to reconcile that hell with the current city. I couldn’t understand: How were people over it?”

Sabath’s crying became incessant thereafter. She couldn’t sleep; each time she saw a plane in the air, she panicked, just as her grandmother continued to do. “My mother ended up taking me to a witch doctor,” she told me. “They thought I was haunted by the ghosts of Hiroshima” (called yurie, or faint spirits). For years, the yurie resurfaced in Sabath each summer, making her anxious, watchful, her eyes skyward.

In recent years, a growing body of scholarship has sought to better understand accounts like Shoji’s and Sabath’s through the framework of “trans-generational trauma,” which traces experiences of catastrophic loss across the span of a family or a community. A wide range of studies have examined evidence of “secondary trauma” in the children of Holocaust survivors, the wives of Vietnam veterans, and, more informally, in the families of U.S. veterans who’ve faced P.T.S.D. after deployments to Iraq and Afghanistan. In 2007, a study on the wives of fifty-six traumatized war veterans in Croatia found that more than a third of the veterans’ wives met the criteria for secondary traumatic stress; often, this meant symptoms “similar to those present in directly traumatized persons: nightmares about the person who was directly traumatized, insomnia, loss of interest, irritability, chronic fatigue, and changes in self-perception, perception of one’s own life, and of other people.” More recently, speaking to Mac McClelland for an article on trauma in the families of Iraq and Afghanistan war veterans, the clinical psychologist Robert Motta said, “Trauma is really not something that happens to an individual.” Instead, he proposed, “Trauma is a contagious disease; it affects everyone that has close contact with a traumatized person.”

But even metaphors of trauma as contagion feel inadequate, or even potentially counterproductive; for one thing, they can get mixed up with questions of shame and stigma, seeming to assign blame or stir up anxieties about contamination where the antidote to both is needed. And stigma, too, gets internalized. As a small child, Sabath said, when she began to fear a plane above, “I would think, how could I let the plane know that I was American?” She would beg her father to come along to Japan during the summers, thinking, “My white military dad—a Navy JAG officer—he signalled my identity, my patriotic Americanness.” Only in his presence could she feel, as the mixed-race grandchild of a hibakusha, that “there is no way you would ever harm us.”

When she reached high school, Sabath became a debating champion and made nuclear proliferation her focus. She went on to college at Yale and visited the White House as a student leader for Global Zero, the international nuclear-disarmament group, for which she recently authored a personal essay on herbachan’s “scenes of living hell.” “I hope you will remember my grandmother’s message and act upon it,” she wrote.

* * *

In the late nineteen-fifties, the Japanese government began issuing certificates to hibakusha, entitling them to certain health benefits, and Shoji became the first survivor living abroad to travel back to Japan to reap the benefits. Over the course of those treatments, Shoji gathered for the first time with other survivors, at healing hot springs. It was in that community that she got her first glimpse of psychological relief, and perhaps began to decipher some of her experiences and speak of them to others. Last fall, she traveled to Yale to say to her granddaughter’s classmates, “I want with every breath, with all my strength, to tell people” about the bomb.

In the final pages of Hersey’s “Hiroshima,” he observed that many people he met there were often reticent to speak or even think about the ethics of the bomb; instead, they would offer approximations of “Shikata ga nai,” a Japanese expression that he translated as “It can’t be helped. Oh well. Too bad.”

At eighty-eight, Shoji seems to have thrown off that cosmic shrug. When we finished in the dining room, her daughter gave me a bundle of pastries and fruit, and we all shuffled to the foyer. The whole family stood in the doorway and waved goodbye. Shoji’s cheeks looked pink, and, as I drove off, it was easy to imagine how she might have appeared on her way to work at nineteen, looking up at the August sky.

Sarah Stillman is a staff writer at The New Yorker and a visiting scholar at the N.Y.U. Arthur L. Carter Journalism Institute.