What is the relationship between plaque, dental caries (cavities), and gum disease?

There are thousands of different kinds of bacteria in the plaque on your teeth. Some kinds of bacteria cause dental caries; other kinds of bacteria cause gum disease. There is also the "good" kind of bacteria called indigenous microflora that you need inside your mouth to fight off the "bad" bacteria. Dental caries are not just caused by plaque alone. It is the combination of sugar and plaque that forms an acid and causes cavities.
The first stage of gum disease is bleeding gums. This is also the only stage that can be totally reversed. The bacteria in the plaque on your teeth causes gum disease. If nothing is done to get rid of this first stage of gum disease, also known as gingivitis, then it will progress into periodontal disease, which is not reversible because it involves the loss of bone support around the teeth.
None of these conditions can occur without the presence of bacterial plaque inside the mouth which is why it is important to be very meticulous about daily homecare of the teeth and gums. Research has shown that gum disease is directly related to heart disease and other overall health issues.

Plaque causes cavities... If your not flossing daily the food and plaque build up in between your teeth causing cavities. Not flossing/brushing daily leads to gum disease. And you always should see a dentist at least every 6mos to get your teeth cleaned to remove the hardened plaque called calculus.