Contrary to popular belief, you don't get sick the minute a virus enters your body! Viral Load is the amount of virus in a patient. When the viral load is low(at the beginning and end of infection with a virus) a patient may be completely asymptomatic.
Thats why so many diseases, like covid 19, have a beginning period of time that is relatively symptom free. It doesn’t mean you’re not infected, it means that there is just not enough viral particles in your body yet to trigger your immune system!
Your immune system has trouble recognizing a problematic foreign body until it begins to do damage in the body. Covid-19 takes about two weeks to show symptoms because it takes about that long, typically, for the amount of covid-19 virus particles in the body to trigger a reaction from your body!
You can still pass the virus before you have symptoms though! because you can have millions of particles in your body before the symptoms show!
Thats why quarantining is so important, especially if you MAY have been infected, not just if you feel sick