To avoid re-infecting yourself DURING and after a cold/flu, soak your toothbrush in rubbing alcohol to kill the germs.
Toothbrushes can re-infect people with the same cold/flu they have just gotten over. Wash toothbrushes in the dishwasher after being sick.
To prevent colds and flu, parents and kids should not share tubes of toothpaste. Each family member should have their own tube of toothpaste.