Tooth decay is a term used to describe teeth that are rotting.  The rot can sometimes be hidden deep within a tooth and lay  unseen until the tooth either collapses, pain develops or your dentist spots it.  Only a dentist can fix tooth decay once it starts going however there are factors that you can control to help keep the risk of decay to a minimum.

	It may sound obvious but if you limit the time that your teeth are exposed to bacteria and food then this wil be benificial. Brushing last thing at night and avoiding snacking helps to keep teeth healthy.
In order for bacteria to rot teeth you have to have teeth. Even if you don't, remember to brush your dentures and mouth.  Bad breath is often caused by large accumulations of bacteria. Infants without teeth should still not be given sugary treats as they will demand it once the delicate baby teeth erupt.
	To put it simply, lots of bacteria produce lots of acids.  If you can see a white film on any part of your teeth and it scrapes off with your fingernail then you should really be brushing again. That is bacteria. As the film gets thicker, more damage occurs to the teeth.
Bacteria needs a food source and they can get this from things you eat.  Tiny food particles in the mouth provide a means for the bacteria to grow and multiply. Rinsing your mouth out with water after eating can help to dislodge food particles and  also neutralises the acids that cause tooth decay.