As Herr Drumpf said about an aide to Pence that caught the virus:

“She tested very good for a long period of time, and then all of a sudden, today she tested positive. … So she tested positive out of the blue. This is why the whole concept of tests aren’t necessarily great. The tests are perfect, but something can happen between a test — where it’s good, and then something happens, and all of a sudden — she was tested very recently and tested negative. And then today, I guess, for some reason, she tested positive.”
?
It’s a mystery!