When the NFL and NFLPA were negotiating COVID precautions for this season, I believe the NFLPA was in favor of daily testing of all players. What was decided upon, however, was weekly testing. The NFLPA's idea was basically that if a player was tested Monday, he could contract COVID Tuesday, practice and play that weekend's game all while being positive, and not turn up negative until the following Monday. In hindsight, that would have been a better idea. It's almost impossible to believe that every one of the players testing positive contracted COVID from outside sources. It's likely that many/most of them have been spreading it to each other, without knowing.