Carbon monoxide poisoning.
The classic "silent killer" household riddle.
Everyone gets hit with the same flu-like symptoms (headache, nausea, dizziness, confusion, fatigue) at once, but they magically improve as soon as you step outside. CO from faulty heaters, stoves, or generators binds to hemoglobin 200–250× stronger than oxygen, starving tissues while leaving SpO2 readings deceptively normal