I think people with a functioning immune system should not attempt to limit their exposure to microorganisms (except in the obvious cases like being in Liberia half a year ago). It's both useless and counterproductive.
I tend to think so too, but
there are people with very varying strengths of immune systems
the strength of the immune system changes over time (I notice that older people both tend to be ill less often and also to be more cautions regarding infections)
handshakes are a strong social protocol that not everybody can evade easily.
you could still intentionally expose yourself to microorganisms
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