For that matter, how could one of them show no symptoms (asymptomatic), while the other could be floored for days?
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Two reasons.
First, being sick isn’t and on or off switch, there’s levels of intensity. Someone could have the minimum amount of virus cells in their body to show positive on a test, while the other person could have ten times more cells of the virus.
Second, two people who are just as healthy can have different reactions based on genetics. One person could have very over-active cells that work so hard to fight the infection it ends up hurting the person. One person could have an under-active system and by the time they take medicine the virus has spread further than normal.
If someone gets just a little sick, has good genes, and a good lifestyle, they could be asymptomatic but still pass the virus onto others.
Your immune system is determined by your genetics more than your lifestyle. Even if two people live similar lifestyles, their immune systems could react very differently to the same threat (any given disease).
Remember, some people’s immune systems launch an all-out mega response to harmless things like peanuts – that’s what allergies are. Immune system over-reacting to something harmless. And the reason some people’s immune systems do that isn’t their lifestyle.*
*Yes under-exposure to germs as an infant predisposes you to worse allergies later, but that’s not the point and beyond ELI5. Me and a co-worker can have the exact same lifestyle for the last 10 years yet my peanut butter sandwich would kill him. People’s immune systems differ wildly due to genetics.
Lots of factors. If most things like genetics, lifestyle, environment, diet, physical are equal…
with covid they believed how much of the virus was taken in at the time, and where it landed. (lungs, throat, nasal passage..thought to be worse for long term covid)
it was believed that people who suffered colds in the last few years had some antibodies against the corona family.
IIRC the working theory was that the immune system has antibodies floating around to deal with everyday issues. We have a storage system for unused antibodies. When we face a different virus we pull out other tools for the storage to find something that works. That takes awhile so in the meantime the virus can spread faster than the body can respond.
Consider the old saw about kids who play in dirt are healthier, this is probably why.
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