Gum Disease

The bacteria around and in between your teeth give off toxins that can cause the gums to swell and turn red.  Basically, they are like having a toxic waste dump living in and around your teeth and gums. 
Over a decade or so of exposure to this toxic waste dump around your teeth and gums, the bone between the teeth will start to painless and permanently erode away.  This is known as gum disease, periodontal disease, or pyorrhea.
Bacteria are also the source of bad breath.
If you take a McBurger and throw it in the back of the car for a few days, your car will soon smell like rotting McBurger.  Leave a chunk of the burger between your teeth for a few days, and what do you think happens?
This is why it is recommended that you brush and floss at least once a day.
Be sure to feel the brush against your gums when you brush. It is not necessary to always use toothpaste when you brush.  If you wish, put a brush and toothpick in some nook of the dash of your car and brush and floss at stop lights.