Why do you gain resistance to some illnesses after having them but not others?
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The usual answer is you are not catching the same thing again, it tends to be a little bit different from the last strain. (like how people get a flu shot each year because the virus itself changes from year to year).
There are a myriad of other reasons of course, but I think this is the most basic reason.
It has to do with how your immune system deals with diseases, in the case of viruses or bacteria your immune system can usually store their markers for easy identification later. How long they stay stored depends.
Moreover a disease like the common cold changes it’s markers so quickly that the next time you get it the information your immune system has is already outdated.
Then there are diseases like cancer that don’t involve a bacteria or virus at all. Your immune system can’t store the markers here because there are none.
There are two explanations I know of.
Diseases mutate over time, some faster than others. Sometimes they mutate in a way that your immune system doesn’t recognize them as the same disease again.
The other reason I know is that we simply don’t know. Our body is very complex and there are things we still don’t understand. Some anti bodies just stay longer in our blood than others and we don’t know why yet.
tl;dr nobody knows (yet)
You gain a resistance to every virus, bacteria, and fungus that your body encounters. Sometimes you gain a really strong resistance that lasts for a lifetime (like measles) and sometimes you get a really pathetic resistance that wears away every two years (like RSV). Sometimes you don’t become immune, but the second infection is way less likely to kill you (like malaria or COVID-19). Sometimes you become mostly immune, but the disease is really good at hiding from your immune system so it establishes a chronic infection, like hepatitis C, HIV, or syphilis.
In some cases, we know the reason why. HIV and herpesviruses stay around forever because they hide inside of your cells and live there forever, so even if your immune system gets good at fighting, it just can’t eradicate every last virion. In some cases the reason is unclear. Immunity to RSV is very short-lived and I don’t think anybody knows why. Or for the flu, it’s because the flu mutates very quickly.
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