A dental abscess occurs when the bacteria that cause tooth decay (caries) reaches the nerve and blood supply in the tooth. The bacteria can then destroy bone tissue and cause a swelling of the gums before the pus and bacterial byproducts burst into the mouth. A dental abscess may often appear as a "gum boil" in early stages but can progress to facial swelling (cellulitis) which can lead to a true medical emergency. Seeking immediate evaluation is very important if you suspect a dental abscess.