Pertussis bacteria can be found in the respiratory tract when many kinds of chest conditions are being investigated, but they do not seem to be involved in these conditions. When however they find the lungs of a very young baby it is a different story.
Toxins are produced that cause blood cells to clump together and block the lungs, leading to oxygen deprivation and brain injury or death in many cases.
Older individuals encountering these bacteria for the first time usually escape the blood problem but may get the severe coughing spasms (described fully on this site) if the bacteria manage to take a hold in their new host.
Pertussis immunisation gives really good protection against the blood problem that infants experience, but is not so good at protecting against the cough. Until 2012, babies still died from pertussis because it was not possible to start immunising them until they had passed their most vulnerable time (first 3 to 6 months). It was then found that boosting the mother’s immunity during the pregnancy with pertussis vaccine gave almost 100% protection during the vulnerable period. A perfect solution indeed!
Children start to get their own pertussis vaccine shots in their first 6 months and this gives them very good protection against whooping cough, which can be serious for young children, but not as serious generally as for babies.
The vaccine does not give perfect protection and it most commonly occurs in teens and adults, when it is usually not recognised for what it is, putting young infants a great risk.
If this is the case, people ask, why not continue to give booster shots throughout life, indeed, some countries recommend it every 10 years.
Unfortunately this does not have the hoped for benefit because we are constantly coming into contact with pertussis bacteria and they periodically boost our immunity without causing illness, so extra shots make little difference once we have completed a basic course.
Given this knowledge about our constant exposure to this bacterium it remains a bit of a mystery why is causes peaks every 4 years or so. It seems unlikely that our collective immunity drops in everyone in that cyclical way. It is more likely, it seems to me, that there is some other factor at work, perhaps another infection, that triggers the proliferation of pertussis in such numbers that they start to damage our tissues and cause symptoms, that then cause a big spreading situation.