An insect was living in the bristles of their toothbrush
Brushing your teeth before you go to sleep is very important. No matter how tired you are, you should always make sure not to skip your oral hygiene routine. But one thing you should never do is share your toothbrush.
Especially not with a spider. We cannot imagine how gross it must have been for this person to realize there was an insect on their brush. Maybe just gargling with some mouthwash will have to do this time.