Just take Measles for example.
Measles can cause immune amnesia after an infection. Infected memory T and B cells get destroyed by your body's immune response, essentially resetting your immunity. You can lose up to 80% of your antibodies. You end up with good immunity to Measles, but you lose immunity to other pathogens.
It normally takes a child 3 years to recover the lost immunity as they have to get infected/vaccinated again for all those previous viruses. In many cases such children die of those subsequent infections years later.
The epidemiological impact of this effect was massive. Researchers estimated this immune amnesia was partly responsible for half of childhood infectious disease deaths.