This bacteria can lead to all kinds of infections, so changing your brush on a regular basis is one way of preventing that from happening. The scary part is that research has shown that up to 10 million bacteria can actually live on a single tooth brush and that is going to lead to a high probability of you being able to pick up some kind of infection. This is made worse if you already have damaged gums or a mouth ulcer as that gives the bacteria an opening to take advantage of and it is certainly not going to be shy in doing that.
It removes the chances of other bugs getting into your system.
This point is more aimed at when you have had the common cold, a cough, or flu because if that is the case, then it is in your best interest to change your tooth brush no matter how old it may be. The reason for this is because there can still be aspects of the virus on your brush, as you have hopefully still been brushing even when ill, and it could lead to you taking the virus back into your system and it making you ill once again. This is especially true if you already have issues with your mouth as your immune system will be lower and more likely to fall victim to the bacteria on the tooth brush. It is such a simple thing to change it in order to prevent it from happening that it is difficult to see why anybody would not do it in the first place. Interestingly enough, the tooth brush itself is not protected from bacteria when you first buy it, which is why you should soak it in antiseptic mouthwash before you even use it. It is certainly not wise to just start brushing out of the box.