Many diseases are most contagious just before or as you are just starting to be sick.
But working on the presumption that everyone is in that category is absolutely foolhardy and completely unrealistic.
I am not going to walk around presuming I am sick just to placate someone who is paranoid or wants to control me so bad. Forget that.
We used to call that pre-symptomatic in the days before SARS-CoV-2.