I was scared of pertussis when my baby was under 6 months. But now I'm not afraid of it at all. I've got my SA in the cabinet waiting in case I need it.
Like other said, I'm more afraid of the aP vaccine than the disease itself - even in a younger than 6 month baby (and I'm still scared of the disease in an <6 month baby).
I am certain I had pertussis in college, for about a year. I had a horrible, horrible croupy cough, which in hindsight was very 'whoopy' but I didn't know then that I could get whooping cough as an adult. I saw several doctors and was tested for tb, asthma, pneumonia, LUNG CANCER, and yet never was I tested for pertussis. Because, alas, I was vaccinated, is my guess.