A 4-month-old infant is brought to the clinic with a 2-week…

A 4-month-old infant is brought to the clinic with a 2-week history of worsening cough. The parents describe episodes of rapid coughing followed by an inspiratory “whoop” and occasional vomiting. The infant is feeding fairly well between episodes and has no signs of respiratory distress. Which of the following is the most likely diagnosis?