Why do we develop immunity to a virus when exposed to it but not bacteria? I have had measles, mumps, and chicken pox and have immunity for life (?) but I have had strep throat a bunch of times.
A cold is a virus, but you can catch many colds in your lifetime because there are many variations, and they are constantly mutating.
It’s not quite that simple. You have had colds, a viral disease, more often than you have had strep throat.
Conversely you will only have had TB, whooping cough, tetanus or diptheria once, if at all, despite those being bacterial diseases. The reason you probably won’t have had those bacterial diseases at all is because you have been inoculated against them, and that one exposure results in life-long immunity just the same as for measles, mumps and chickenpox.
So as you can see it’s not in any way true to say that we develop immunity to viruses but not bacteria. The truth is that we develop an immunity to any pathogen whether bacterial, viral or otherwise. The question is how long the immunity lasts, and various bugs have found ways of overcoming that immunity.
Some pathogens such as cold viruses evolve so fast that the strain we are immune to has altered into something we don’t recognise within a few months.
Streptococcus pyogenes, the agent responsible for strep throat, has a number of ways of evading the immune system.
S. pyogenes is found in low numbers in the throats of all healthy adults. However once the immune system is weakened or the tissue damaged, usually by infection with a cold virus, the bacteria run wild. That’s why colds are so often accompanied by a sore throat, the bacterium is taking advantage of a weakened immune response and damaged tissues to mount an attack.
S. pyogenes also ‘hides’ from the immune system by only inhabiting the outer layers of the membranes of the nose and throat. Those areas have low or non-existent blood supply and so there is no possibility of an immune response. This hiding ability accounts for the recurrence of most bacterial diseases. Viruses don’t have that option because they can only ‘live’ actively growing cells, and actively growing cells need an active blood supply. IOW viruses are obliged to expose themselves to the host immune system at least briefly, bacteria aren’t.
Some cases of strep throat are caused when a particularly nasty new strain of the bacterium is introduced, but in those cases the body does rapidly mount a defence an immunity to that strain develops. Thereafter that strain can only exist in the mouth flora as normal because once it comes into contact with the bloodstream it is eliminated.
S. pyogenes strains that cause strep throat also produce a toxin that bursts red blood cells. This in turn triggers a clotting response that gums up the blood vessels. No blood flow = seriously reduced immune response. The bacteria can then go about there business without worrying about any host immunity.
In summary the body does mount an immune response to bacterial diseases as well as viral diseases. It’s just that many bacterial diseases are caused by bugs that have methods of evading a healthy immune system which viruses, because of their small size and reproductive needs, can’t do.
Hope that answers the question.