There are a couple of factors involved with whooping cough and
immunizations.
1. Pertussis (whooping cough) vaccine is rarely given after 6-7 years of
age. (I'm not sure why - reactions or custom, maybe someone else knows)
2. Therefore as you get older (sorry Pearl) your immunity wanes.
3. Adults are the biggest pool of whooping cough carriers today.  It is
frequently not dx because adults don't usually have the characteristic
whoop, just a NASTY lingering cough.  There is talk of doing adult
immunizations now that we have accelular pertussis, but so far just talk.
I saw my first case of infant (7 weeks old) whooping cough about a year ago
and I really don't care to ever see another!  Actually we ended up having 3
in our inner city practice between June & Oct 1996. It is big time scarey
stuff.  Sincerely, Pat in SNJ