Final answer:
Pertussis, or whooping cough, occurs when there is mucus accumulation in the lungs, leading to severe coughing episodes. It is most common in infants and children.
Step-by-step explanation:
Pertussis, commonly known as whooping cough, is caused by Bordetella pertussis, a gram-negative coccobacillus. It occurs when there is mucus accumulation in the lungs, leading to severe coughing episodes. The coughing can be accompanied by a sound resembling a "whoop" when inhaling, hence the name whooping cough. While adults can be infected, the symptoms are most pronounced in infants and children.