Tooth decay, or dental caries, is caused by prolonged exposure to acids produced by bacteria in mouth. We cannot avoid bacteria in air. When we breathe it enters mouth and attaches itself to everything, including teeth. In fact, there are more microcosms in one mouth than there are people on earth.
Sugar is main culprit in tooth decay, because that is what bacteria eat. Bacteria then produce acids as a byproduct. Those acids eat at enamel of our teeth, until hole or cavity in tooth appears.
Our story begins in 17th century, when sugar plantations developed in “new world.” Until then food was produced and prepared with much less added sugar. Then 18th century saw sugar beets being harvested in England. Now, virtually everything we consume, from cereal in morning to steak and eggs at night contains extra sugar. Bacteria on our teeth count themselves lucky to live in 21st century, where there is an almost unlimited supply of free food for them to thrive on.
We are in cahoots with sugar and bacteria when we do not brush and clean our teeth. Leaving bacteria to feed upon sugar and produce acids in our mouths allows bacteria time to form a visibly organized colony between gums and tooth that we call plaque. Plaque actually acts as a cover for acids that sit on surface of our enamel. Without cleaning, acids will eat at out enamel almost at will, creating tooth decay and dental caries.
Decay is demineralization. In other words, outer tissue of tooth is so hard because it is 95% mineral. The inner tissue of tooth, dentin, is a little softer because it is only 66% mineral. Normally, saliva is a natural remineralizer when acids have begun demineralizing, but when plaque is involved saliva is almost powerless to repair damage. Acids will begin with a little hole in enamel, and once it makes it through to dentin, it eats tooth tissue from inside out. This means serious dental action: fillings, or even a root canal.