First, the good news: Your bathroom is usually one of the cleaner places in your home.
But if you're not cleaning things the right way (or, OK, just being lazy), your bathroom can turn into a breeding ground for germs, bacteria, and viruses.
1. Using the same toothbrush or forgetting to ~deep clean~ it after you've been sick.
2. Or keeping all the toothbrushes together in the same old cup thing.
If you share a bathroom with roommates, cramming all your toothbrushes into a shared holder is basically inevitable. But if it's so crowded that the bristles are touching and transmitting germs, you may as well be sharing toothbrushes with your roomies (and their significant others). And if someone gets sick, it's likely those germs will get on your toothbrush too.
Besides, have you ever looked at the bottom of the toothbrush cup after not washing it in a while? It's like a reservoir for germy mold and mildew. Reynolds suggests storing toothbrushes individually upright or keeping several cups so they have room to breathe. And definitely sanitizing the cup with a diluted hydrogen peroxide solution once every few weeks.