I never understand people who think the immune system is a muscle that can be strengthened for anything by lots of exposure to dirt and germs
It doesn't work that way, the only way to strengthen your immune system at all is to eat well and exercise and not damage your body by smoking or drinking or taking substances that upset the balance.
Balance is individual too, some people are born with no functioning immune system or an immune system that attacks their own body...
The other thing is to make antibodies, you gain antibodies from your mum during pregnancy or breastfeeding or by getting sick/vaccinated from an illness.
Its not guaranteed to make antibodies, which is why anti vaxxers make vaxxers so cross. Vaxxers cannot know 100% that all the carefully given vaccines have had the correct immune response. That's why certain vaccines are repeated in the first years of a child's life, to increase the chance they make antibodies.
Flu vaccine only protects from the flu strains included in it, the most dangerous ones around at the urrent time, not all flu.
The best way to avoid being ill is and always will be good hygiene, taking precautions, being healthy and vaccines