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
There's also a difference between exposing yourself to microorganisms and exposing yourself to high levels of one particular one that is shedding from someone that has already caused them illness.
If it's worth saying, but not worth its own post (even in Discussion), then it goes here.
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