What is the purpose of using vaccines against commonly found bacteria? Since the human immune system develops at birth and shortly thereafter (via colostrum) natural defenses against common bacteria, presumably the body already has antibodies against common bacteria. What purpose is there in "innoculating" someone against such bacteria?
For example, bacteria like pneumococcus endemically colonize everyone's lungs. In one study pneumococci were found in 80% of the healthy patients studied. Of course, that is just the bacteria that were found. Undoubtedly the other 20% of the healthy patients had various colonies of pneumococcus which simply went undiscovered. So, obviously the human body is well adapted to constantly defend against pneumococcus, yet there are varieties of pneumococcal vaccine anyway. Why would this be? To make money, or is there a proven biological action that implies a benefit? By a "biological action" I mean an explanation of the immunological mechanism by which the innoculation significantly augments pre-existing T-cell/B-cell receptors for common bacteria such as pneumococcus, and has a real benefit which is not merely reduplicative.