Our immune system is quite aggressive with garbage collection and can picky about what pathogens it chooses to remember. This behaviour makes sense on the hypot

Our immune system is quite aggressive with garbage collection and can picky about what pathogens it chooses to remember. This behaviour makes sense on the hypothesis that our immune system has a "storage limit" for immunities. It doesn't make sense if the immunity memory is unlimited. (The reason why many vaccines require boosters is that without the repeat exposure, our immune system decides it doesn't need to remember about that particular pathogen.)