A 6-mоnth-оld pаtient presents with repоrts of cough аnd mаlaise. She was sick for two weeks with a runny nose and cough. The cough has worsened in the last two days, and it sometimes causing post tussive emesis. She has "fits" coughing. The cough is sudden and forceful. She has had no fever. This infant is behind on vaccinations. There are no other children at home. What is the most likely diagnosis?