something piqued my interests- if reinfections are so common, how long can natural immunity last?
This question need to be spilt into 2 in order to be clear on what you are asking.
How long can natural immunity last?
Your natural immunity for a certain bacteria or/and virus last a lifetime. You DNA store the information to fight that particular virus and so if you are infected, your immune system starts their protection.
Why reinfections are common?
Your reinfection refers to showing of symptoms, i presume. You can be reinfected and show no symptoms as your immune system is strong enough to do the work or worse is to show mild or very slight symptoms. you recover fast or totally ok.
If you are showing symptoms after reinfection, even though you have immunity from it, you have to ask yourself to check on your diet and lifestyle. Having immunity does not mean you 100% dun show symptoms, especially in the case you eat all kinds of crap food like those found at hawker centers ( hokkien mee, fried kway tiao, Mcd, fries, ice cream) frequently. lifestyle of stress lah. You are creating weak immunity for yourself even though you have natural immunity against that particular virus.