A 6-month-old female infant from rural Maharashtra presents with a 3-week history of paroxysmal cough. The mother reports that the cough is worst at night and is followed by a high-pitched inspiratory sound (whoop). The infant has not received any vaccinations. On examination, she is afebrile, with mild conjunctival injection and scattered crackles on auscultation. A nasopharyngeal swab culture on Bordet–Gengou medium grows small, gram-negative coccobacilli. What is the most likely diagnosis?