Vomiting after a coughing spell or an inspiratory whooping sound on coughing almost doubles the likelihood that the illness is pertussis. The absence of a paroxysmal cough or posttussive emesis makes it almost half as likely.

The illness usually starts with mild respiratory symptoms, including mild coughing, sneezing, or a runny nose (known as the catarrhal stage). After one or two weeks, the cough typically develops into uncontrollable