I was arguing with an anti-vaxxer (clearly a favorite pastime) when he made the argument that there are certain risks, like triggering of autoimmune disease, inherent to the process of the adaptive immune system "learning" a new pathogen (memory B/T cell formation, etc.). His argument is that by administering vaccines, we are causing the body to "learn" new pathogens at several times the normal rate, thereby increasing the number of exposures to these risks.
While I'm thoroughly pro-vaccine, there is some truth to there being risks to "learning" new pathogens, with devastating diseases like MS, lupus, and rheumatoid arthritis commonly triggered by viral infections (though not as commonly as, you know, dying of diphtheria). Human parvovirus B19 for instance has been specifically linked by numerous studies to development of these diseases. I can't counter his argument without knowing more about how often a typical American or European adapts to a new pathogen, but I've found references to this information very difficult to track down in the literature.
So how often does the typical American/European "learn" a new pathogen, specifically meaning the adaptive immune system beginning the process of forming memory immune cells targeted toward a new pathogen? Once a minute? Once a day? Once a year?