I'm pretty certain 'One By One' is pure fiction, though Freida McFadden does love to tap into those 'this could really happen' anxieties. Her books often feel grounded in medical or domestic settings, so they can have that unsettling ring of truth, but I've never seen any evidence this specific story is tied to a real case. She's talked in interviews about drawing inspiration from the general fears surrounding hospitals and trust, not from specific headlines.
What makes it feel so plausible, honestly, is how ordinary the initial setup is. A hospital ward, a patient, a nurse who seems maybe a little too involved—we've all had that tiny, irrational flicker of doubt about a medical professional. McFadden just takes that flicker and fans it into a full-blown inferno. The mechanics of the plot, with the locked-in ward and the escalating tension, are classic thriller architecture, way too neat and contained to be a direct retelling of a true crime.
If it were based on a real event, I think we'd have heard about it by now, either from the author or from true crime circles drawing the connection. Since we haven't, I'm comfortable filing it under 'devilishly clever fiction' that preys on our very real, very common vulnerabilities.