There are a few behavioral markers that indicate something is very wrong.  Him halting tweeting for any appreciable length of time is a big one.  The abrupt departure for the Walter Rees National Military Medical Center with no prior press statement is another.  He went there without Melania.

Contact tracing indicates spread at the Amy Coney Barrett introduction event.  Senators Lee & Tillis are both positive as well as Melania in addition to other staff who were front row at the event.  Symptoms took several days to become apparent per what we know so far.