If “Natural Immunity” was worth a damn, we wouldn’t have overcrowded hospitals.
There’s this common belief, though not usually coherently expressed: if you microdose on diseases, you’ll strengthen your immune system to the point where it can handle anything.
That’s not how it works. Your front-line fighter cells never get stronger no matter how many diseases you face; their effectiveness is based on genetics and your general health. Your special-ops cells have to spend awhile “training” (sort of, it’s complicated) before they can join in the fight, and once they do, they’re only effective against the specific pathogens they’ve trained for. Also, the actual fight is often long over by then, it’s mostly so that they can utterly wreck the next invasion before it can really get started. That’s what vaccines are for, so they can train in advance and join in the fight immediately.