It starts like a cold, with congestion and upper respiratory symptoms, and then progresses to a cough. You have fits of coughing that are so severe that you can't stop or catch your breath.
It's that severe, prolonged cough, and especially the characteristic paroxysms [uncontrolled fits] of cough that trigger physicians to be worried about pertussis and try to confirm the diagnosis.
The symptoms are pretty nonspecific, and so doctors don't always suspect it. Pertussis is high on the list if that whoop is present. The “whoop” sounds like a sharp gasping intake of breath after all the air has been coughed out of your lungs. If it's not, it's likely to go unrecognized because there aren't really other signs and symptoms that are as characteristic.