Thank you for the link MOnica.
I think, from what I was taught, it is very hard to generalise about the immune system, because individuals vary immensely in their ability to withstand infection.
A healthy adult immune system is exposed to millions of viruses and bacteria each day, just by living, unless in a sterile bubble.
General health physical and mental, diet, gut health (microbiome) smoking, drinking, so many factors play a part.
Some individuals are very vulnerable to infection because of illness, drugs and so on.
It makes sense to me to mitigate against unnecessary exposure to multiple circulating infections just now, including covid, which may put me in bed for a couple of weeks, by wearing FFP mask in crowded situations such as supermarkets.
This doesn’t restrict my life, because I am still mixing and socialising, and I believe my immune system is having plenty of challenges, but can’t work as efficiently now I am older. Best defence against harmful pathogens is vaccination if possible, not exposure.