Without sounding stupid,how can the guess people with the virus having mild symptoms
& then infect others but with mild symptoms
There's something called viral load, which is the amount of virus in you at a given point. People with mild symptoms have a low viral load, so they give off low amounts of virus. If you get infected by them you get a really low dose, and then your body can get on top of it and fight it easily - so you have a mild version.
If you are really sick, you have a massive viral load, and you give out big doses of virus, which means if you get infected by them your body has a much bigger fight and can get overwhelmed which makes you have a severe case.