If you're like most people, you get a new toothbrush every six months, but you really should be changing your toothbrush more often, like every three to four months. When the bristles get worn and lose their rigidity, they don't remove plaque as well. But you should also do some maintenance within that three months, when your toothbrush is just sitting out in the open. After all, think about all the germs and bacteria floating around that would love to attach themselves to your moist toothbrush.
So to prevent colds, flu, and other sicknesses, once or twice a week run your toothbrush through the microwave on high for 10 seconds immediately before brushing your teeth. If someone in your home is sick, you may want to sterilize it in the microwave each time you brush. If you're the one who's sick, after the illness passes, ditch your old toothbrush and replace it with a new one