Final answer:
The catarrhal stage is the most infectious stage of pertussis, featuring mild symptoms but a high potential for transmission of the B. pertussis bacterium.
Step-by-step explanation:
The most infectious stage of pertussis is the catarrhal stage, which is the initial phase of infection. During this stage, symptoms may include nasal congestion, a runny nose, sneezing, and a low-grade fever. However, it is in this phase that B. pertussis, the bacterium causing whooping cough, is most transmissible to others. After the catarrhal stage, the disease progresses to the paroxysmal stage, which is characterized by severe coughing spells. Ultimately, the infection enters the convalescent stage, where the symptoms begin to decline, but the cough may persist for a longer period. Understanding when pertussis is most contagious can help in preventing its spread.