Carrie, of course, is not otherwise healthy. She's prone to seizures, one of the rare complications of pertussis. She also has that problem with swallowing.
Think back to the last time you had a bad coughing fit. It's gross, but think about the saliva and the mucus. Think about the last time you threw up. Now imagine all that together, along with gasping for air--and not being able to swallow.
If Carrie gets pertussis, she'll almost certainly get pneumonia. Well, she will if the doctors can keep her from choking to death first. There's a very good reason that pneumonia is a common complication of pertussis, even without Carrie's problems.
A far too personal vaccination story, at Quiche Moraine.