Also if you look at the time you can be A symtomatic,as in the case of one guy returning from the US,it was incubated in his body for 11 days prior to first symtoms.
So he went to get tested,because his first symtom was a headache that went on for a few days,they wouldn't test him because he didn't have a fever,but as other symptoms emerged,tested again he was positive,so all that time (11 days) he could have passed it to many. The seminar he returned from in the US had 40,000 attendees.