What might happen if you leave a cavity in your mouth without treating it? Tooth decay is like an infection. It is made of harmful microscopic bugs that use sugar to attack teeth. With time this creates a little hole in a tooth, and if not repaired with a filling, the cavity continues to grow.
If a cavity is not treated it can destroy a good part of the tooth, making it hard to be rapaired with a conventional filling. If a lot of the tooth is gone, only a crown can fix it.
If the decay’s micro-organisms reach the pulp chamber, where the nerve and blood vessels are located, then the pulp becomes irritated and infected. This can eventually lead to an abscess which can be very painful. Only a root canal can fix a tooth when its pulp is infected, and a crown is then probably needed as a final restoration.
There are also situations where a tooth is so much destroyed by a cavity that nothing can be done to fix it, not even a root canal and a crown. In that case the tooth would sadly need to be extracted.