Final answer:
Pertussis has three clinical phases: catarrhal, paroxysmal, and convalescent.
Step-by-step explanation:
The three clinical phases of pertussis are catarrhal, paroxysmal, and convalescent. The catarrhal phase is the initial stage of the infection and is characterized by mild symptoms such as a runny nose, nasal congestion, and a low-grade fever. The paroxysmal phase is the most serious stage, marked by severe and prolonged coughing spells. The convalescent phase is the final stage, during which the cough gradually subsides and the patient recovers.