This person’s filling was leaking, but was not apparent on the xray. If there is fast movement of decay, it can take a while before the decay makes the tooth stucture dissolve and wash out of the tooth. So a hiding environment, say under a filling or in a thin crack, and with the right diet and lack of,or difficulty of cleaning, and the bacteria won’t hold back. 

The most important aspect of root canal treatment to ensure no more bacteria can enter the root canal space. Once the decay is removed from under the filling, the root canal space and its walls are cleaned via files and chemicals. This is done to remove dead tissue, bacteria and any irritant chemicals produced by the bacteria. As there is no blood supply in the root space,we are now doing the job of the body’s immune system.