Just some decades ago if you had a badly diseased tooth, there was no way to save it. Today, with root canal treatment there is a chance to save that tooth.
Each tooth has pulp and nerve. The nerve has been there since the tooth originally formed, but once the tooth has been in the mouth for some time, it does not have any necessary functionality.  

When a tooth has badly decayed or is cracked, bacteria can enter the pulp. If left without treatment, pus builds up at the tip of the root and in-between jawbone and forms puss-pockets, called abscess. This abscess can cause the tooth pulp to die. If the infected pulp is not treated, it causes pain and swelling and eventually the tooth might have to be removed.

Our East Los Angeles dentist can treat a decayed tooth by removing the infected pulp and then cleaning and sealing the pulp chamber and root canal(s). In order to strengthen what is left of the tooth, sometimes a crown is placed. If you continue with your regular brushing and flossing routine and regular checkups and the roots of the restored tooth are nourished by the tissue surrounding them, your tooth can last a lifelong.