Pertussis is cyclical. Meaning, every so many years, there are more cases, and then there are fewer cases again. The Pertussis vax does not confer lasting immunity. It is debatable even whether it confers adequate immunity at all. Even if you've been vaxed for pertussis, chances are very good that you've developed a natural immunity, because the vax-induced immunity doesn't last a lifetime. If you went in to a Dr. with symptoms of pertussis, you would probably not be dx'ed with it if your records state that you were vaxed for it. You would most likely receive a different dx.
My oldest is almost 9, btw, and we've never had anything, despite the fact that we are around lots of people. We certainly don't shelter our children from anything; quite the contrary, I'd love for them to be exposed to some of those childhood diseases now, so that they have a lasting immunity. Many diseases are much milder if contracted as children (think chicken pox).
It is possible, however, to be exposed to the disease, and not develop clinical symptoms, esp. if you are very healthy. That just means that you have a very good immune system, that it is functioning properly.
It can be hard to break away from the fact that vaxes aren't the saving grace of modern medicine. There is really so much that is not known about how our bodies function that to inject foreign substances into it willy-nilly is, IMO, irresponsible. Do your research, take your time, and make an informed decision. There is no rush to vax your baby, you can always catch up if you decide to vax. You can't however, take out what you've put in.
Please go check out the vax forum. It can be intimidating, yes, but there is so much information there that you just won't find anywhere else. If you are really looking for help, you won't find mamas who try harder to help you anywhere else. I can tell you that from my own experience!