Discoveries about the nature of B. pertussis in the the last couple of decades have completely changed our understanding of it. We now know it has a dual ‘personality’. B. pertussis has two kinds of life. 

Life number one causes whooping cough and this website is about life number one. Life number two temporarily invades  our nose and throat but causes no symptoms or symptoms that are minor and are  generally ignored. 

This second life that we do not notice, is 5 to 20 times more common than the first kind. There may be a middle group with symptoms but without any of the characteristic prolonged coughing bouts that are the usual  mark of whooping cough, but the size of this middle group is speculative.

When B pertussis gets into our bodies it sticks to the microscopic hair-like fronds (cilia) that line the large air passages and starts to multiply and produce toxic substances that can damage cells such as white blood cells and also cause the characteristic cough. If we have no immunity these substances can do a lot of damage and even kill very young babies. If we are a bit older than that, these substances can give us what we call whooping cough which is very unpleasant and long lasting. But not everyone gets it severely, and some get it hardly at all for reasons that are not understood yet. However, it is so infectious that we will all get infected in childhood or early adulthood if we have not been immunized. Even if we have been immunized we will still likely get it but probably without symtoms because the immunization neutralises the toxins. Getting infected boosts our immunity even without symptoms.

The immunity we get from the natural infection may last about 15 years but throughout life it can get boosted perhaps every few years by unnoticed re-infection, so we are kept free of the actual whooping cough.

The acellular vaccines that are current in use, and have been for about 20 years, do not give protection for as long as the old whole cell vaccine or the natural infection and do not stop the pertussis bacteria reproducing in our airways so possibly allow it to be passed on. That may be why there seems to be more whooping cough about.

Much effort is being directed at producing improved vaccine but it may be many years off. 
We need to get an understanding of why it becomes severe in some people when it reinfects.