Humans are the only reservoir for B. pertussis. It is spread by aerosolized droplets from the coughing of infected people. Once it finds a new host, the microorganism attaches to and damages the ciliated respiratory epithelium.

Premature infants and patients with underlying cardiac, respiratory, or neurologic disease are at higher risk for complications, which include pneumonia, seizures, encephalopathy, and death. More common complications include epistaxis, vomiting, subconjunctival hemorrhages, syncope, insomnia, incontinence, and rib fractures. Compared with older children and adults, infants tend to have more severe symptoms, develop complications, and require hospitalization.

Typically, pertussis is a 6-week disease that is divided into three stages lasting 1 to 2 weeks each. During the first stage, pertussis mimics common upper respiratory infections, with nasal congestion, rhinorrhea, sneezing, and tearing. The second stage is when the paroxysmal coughing usually starts, with episodes potentially lasting several minutes. In older infants and toddlers, the coughing episodes are followed by a loud “whoop.” Younger infants may have apneic episodes and are at risk for exhaustion. Cough-induced vomiting is also common. In the final stage, patients have a chronic cough, which may last for weeks.