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.
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