You get characteristic GI symptoms.

CNN: "The Secret Life of Germs" author Philip Tierno

Parents and doctors can often miss the initial symptoms of pertussis in the youngest patients because they often do not have the characteristic cough with a "whoop" says Patti.

CNN: 10 infants dead in California whooping cough outbreak

The main symptoms are severe coughing fits which, in babies and children, are accompanied by the characteristic "whoop" sound as the child gasps for breath after coughing.

BBC: Whooping cough cases increase in England and Wales