Final answer:
Pertussis infection has three stages: catarrhal, paroxysmal, and convalescence. The catarrhal stage is mild but highly infectious. The paroxysmal stage is the most serious with severe coughing spells. The convalescence stage is the recovery stage marked by a chronic cough.
Step-by-step explanation:
A pertussis infection can be divided into three distinct stages. The first stage, called the catarrhal stage, is mild and characterized by symptoms like nasal congestion, runny nose, sneezing, and low-grade fever. This is also the most infectious stage. The second stage, known as the paroxysmal stage, is the most serious and is marked by severe and prolonged coughing spells. The final stage, called the convalescence stage, is the recovery stage characterized by a chronic cough.