Worry itself is a normal, even adaptive, function — it's the brain rehearsing potential problems so you're prepared for them, and a healthy amount of it improves performance and planning. Clinical anxiety disorders are defined not by the presence of worry but by three specific features: the worry is excessive relative to the actual situation, it's difficult to control once it starts, and it causes real impairment — missed work, avoided relationships, disrupted sleep — rather than just discomfort. The line isn't about feeling anxious at all; everyone does. It's about whether the anxiety has stopped being useful information and started actively costing you function.
The physical overlap between normal stress and clinical anxiety (racing heart, tight chest, difficulty concentrating) is part of why people often don't recognize when they've crossed from one into the other — the sensations feel continuous even as the underlying pattern becomes qualitatively different. A practical rule of thumb clinicians often use: if the worry is proportional to the actual stakes and resolves once the situation resolves, it's likely normal stress; if it persists regardless of outcome, generalizes to unrelated situations, or actively prevents you from doing things you'd otherwise want to do, it's worth an actual conversation with a professional rather than continuing to manage it alone.
Van der Kolk describes Tom, a Vietnam veteran and his first patient as a new staff psychiatrist at the Boston VA Clinic in 1978. Tom couldn't attend his own sister's Fourth of July barbecue — the noise, heat, and fireworks flooded him with flashbacks of the ambush that killed members of his platoon. He'd stay away drinking instead, then apologize to his wife and two sons the next morning for a rage he couldn't explain or control the night before. His fear wasn't really about anything happening in the present; it was his nervous system's alarm system, calibrated by a war a decade earlier, firing at full intensity in a completely safe backyard.