So repeated infections then.
Seriously, vaccines aside because they prime the immune system, the only way to build up a 'healthy' immune system is to catch multiple infections and fight them off.
It's not a lot of fun, and it does otherwise grind you down, and in some instances can truly be deadly.
So, Polio, Diphtheria, Measles, etc., are still given as vaccines to children, and adults if required.
I am of a generation that bears scars from smallpox vaccinations. We don't do that to children any more, because we all bear scars.......
Can't see that happening nowadays though, too many won't take the vaccinations, so even the horrors of polio are slowly rearing their detestable heads again.
Scarlet fever's back with a bang too.
"Coughs and sneezes spread diseases"....said the old public service announcements.
But those are considered 'demeaning, infantilising and insulting to others intelligence' these days, so aren't broadcast any more.
I feel sorry for those who are immune compromised, whose immune system is so impaired that they don't fight off infection easily.
Thoughtless carelessness is literally killing them.