We still don't know what we don't know.
Here's an example of how training with 1 unknown positive player for 1 hour caused a problem for FC Dallas:
After ‘absolute chaos’ in Orlando, FC Dallas can’t wait to get started again
After receiving their results, the two FCD players who tested positive were immediately put into isolation, as required by MLS protocols. The club and league then performed contact tracing, and all individuals who were in close contact with the infected players began to be monitored. On Sunday, June 28th, the day after arriving, the entire Dallas delegation was tested for a second time.
The team had scheduled its first Orlando training session for that evening.
At the time, MLS protocols allowed all players and staff members who tested negative upon arrival to participate in all approved activities, even if other individuals in their club’s delegation had tested positive in the initial exam. That meant that Dallas could train that Sunday night — and that the team didn’t need to wait for the results from their tests on Sunday morning before taking the field.
Unbeknownst to them before training, a player had tested positive.
...
“I think training started at like 7:30 p.m., we got the results back for the tests at 8:30 p.m., and the guy who had tested positive in that round got pulled from the end of practice and basically got quarantined, brought home in a different way then the rest of the team,” said Hollingshead. “But at that point, we’d all been exposed to him for
about an hour of a practice that we were sanctioned to go take part in.”
With that, three FC Dallas players had tested positive in Orlando; obviously not ideal, but If the spread didn’t continue, the team still had more than enough healthy players to compete in the tournament. The entire delegation was tested again on Monday. This time, all results came back negative. FCD, which had waited for the results before starting practice, was hopeful at that evening’s training.
Once again, their optimism was short-lived.
Three more players tested positive on Tuesday, June 30th, prompting the club to stop training and put its entire delegation in a full quarantine. On Wednesday, July 1st, another three players and one coach tested positive, bringing the total number of positive cases to nine players and one coach.