President Trump doesn’t talk about his grandfather’s death, and he is hardly the only descendant of a victim of the 1918 pandemic who seems to be unaware of that part of his family history. Until recently, at least, the world had largely forgotten the 1918 flu pandemic, even though it took more American lives than World War I, World War II, and the Vietnam War combined. There were few novels or films chronicling the experience then, and there have been few since. Most newspapers and radio stations were slow to report on it.

“It’s really weird,” said Nancy K. Bristow, a history professor at the University of Puget Sound and the author of “American Pandemic: The Lost Worlds of the 1918 Influenza Epidemic.” “There was a complete silencing of that narrative of trauma. It feels so tragic because people’s lives were torn apart by this.”

Among those people were the family of Frederick Trump, whose death came early in the “curve,” at a time when no one fully realized that they were in the midst of a pandemic. New York’s close living quarters, its location as a shipping center, and its position as a hub for soldiers during World War I made it an ideal cesspool for the flu’s spread, but many doctors dismissed the early cases, often thinking that they were routine ailments; it was an era when deadly disease was a more common part of life. And yet, looking back, Frederick Trump’s death was a signature of that pandemic, which not only hit both the young and the old, but also many people like him, seemingly in their prime, healthy middle years.

It hit his son Fred — Donald Trump’s father — especially hard. It was “so immediate, he couldn’t take it in,” said Gwenda Blair, an adjunct professor at Columbia University and the author of “The Trumps: Three Generations of Builders and a President.” Ms. Blair interviewed Fred Trump about his father’s passing in 1991. She was among the few biographers to have had access to the family and to have investigated their genealogical history. “He was very matter of fact about it,” she said.

“It just didn’t seem real,” Fred Trump told her. “‘I wasn’t that upset. You know how kids are. But I got upset watching my mother crying and being so sad. It was seeing her that made me feel bad, not my own feelings about what had happened.’” (Five days after Frederick Trump’s death, his brother in law, Fred Schuster, also died, likely of the flu as well, according to Ms. Blair.)