I don't know why I never thought that keeping your moist toothbrush under a cover after use was a good idea!?! Studies have shown that if you cap your toothbrush, the more bacteria grows.
One thing we don't need more of in our mouths is bacteria.
If you are going to keep going the covered toothbrush route, I would suggest getting a Toothbrush Sanitizer.
Click on the picture above for a link to Brookstone, where they sell sanitizers for around $30.
Not too bad of a deal, if can keep you from getting sick!