[Reki]

There is no scientific evidence for being able to "train your immune system". There is plenty of evidence the opposite happens. Covid especially can cause permanent immune damage.

[godshatter]

We have an innate immune responses and adaptive immune responses. Innate immune responses look for common attributes of certain pathogens and then trigger an adaptive immune response if needed. Adaptive immune responses are highly specific to certain pathogens and can provide long lasting immunity to some of those pathogens, such as measles. How would vaccines work if we only had innate immunity?

So being exposed to certain pathogens can trigger a lasting effect, the length of which depends on the pathogen. If that effect has gone away or lessened severely over many months then it makes sense that only the innate immune response would be available against those pathogens, and the adaptive response would need to be learned again.

https://www.ncbi.nlm.nih.gov/books/NBK21070/

[maxerickson]

For stuff like colds, the adaptive response is typically hampered by genetic change/drift in the virus rather than lessening of the response.

Of course the ability to produce the not quite right antibodies is still often valuable.

[ryathal]

There's actually pretty good evidence that being overly clean and sterile is bad for an immune system. Yes, getting exposed to highly dangerous diseases like Ebola, polio, measles, small pox, etc. is bad, covid with a vaccine doesn't really rise to that level.