The vast majority of dental infections are caused by poor dental hygiene. No matter how you look at it, when teeth are not cleaned and taken care of properly, it leaves room for bacteria to proliferate in various parts of the mouth.
Dental abscess are usually caused by an infection that gets started within a tooth. For example, if you have a cracked or chipped tooth, food particles can get into the tooth and act as a hosting ground for bacteria. Some infections also develop when pulp is left behind during a root canal.
Once they find a place where they will not be attacked by the immune system, it does not take long before infections develop.