Our mouths are home to hundreds, even thousands of species of bacteria. Some of these species feed on the sugars and other carbs in our food, and when they do, they produce acid. This acid attacks our tooth enamel, but the damage is very slight at first. Eventually, the acid can make a small hole in the tooth, a cavity.
Cavities make conditions more favorable for these bacteria. Food gets trapped in the hole, and the bacteria are more sheltered from your saliva, which is harmful to them. And in the cavity, their acid concentrates. This speeds the damage to your teeth.
As the cavity grows larger, you might start to feel sensitivity. Eventually, the cavity will reach the center of the tooth, where the living nerve is. Once bacteria infect the interior of the tooth, it can be very painful–and dangerous. That is when you need root canal therapy.
Filling a cavity in its early stages seals it and slows the damage so you are less likely to need a crown or root canal therapy.