So, people are asking about whether Biden & Trump got tested prior to the debate. And, this is where we get into the slippery nature of tests, and how you can be positive but test negative. False negatives happen a lot, especially early in the infection.
For a test looking for viral DNA to be negative, there isn't enough of the stuff to signal infection... even though the infection might be starting to simmer inside your cells.

You have yet to be transformed into a positive-testing virus factory.
And, if you haven't gotten to the point of shedding virus like cat hair, you might not be massively infectious. There is talk about the amount of viral particles affecting infectivity. But, that said, we do know that presymptomatic people spread COVID.
So, if Trump tested negative because he was really early in his infection and not shedding a lot of virus, he might not have been very infectious himself. But, we don't know.
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