When decay ( a cavity) happens bacteria eat through the out layer of the tooth( enamel). The initial hole is small because enamel is a very hard material and difficult to eat through. However the dentin underlying the enamel is soft and so the cavity spreads laterally once it reaches the dentin. The cavity can more quickly eat through the dentin as it is softer than enamel. Left unchecked the cavity will eventually eat into the nerve causing the death of the nerve. Once this happens the dead nerve tissue turns to pus which leaked out the bottom of the root casing a pis filled hole in the bone under the root. This pus filled hole is an abscess. Once an abscess occurs only a root canal treatment or extraction will cure it.Antibiotics can quiet down the pain but will not cure the abscess.