A tooth abscess differs from a gum abscess by the source of the original infection. The tooth abscess (or periapical abscess) originates from the pulp of the tooth and exits out the tooth's apex at the bottom of the root. A gum abscess (or periodontal abscess) starts in a gum pocket outside of the tooth next to the root.
Click here to see a 239-word sentence that is a perfectly fine sentence (structurally). When two independent clauses are connected by only a comma, they constitute a run-on sentence that is called a comma-splice. The example just above (about the sunscreen) is a comma-splice.
A comma splice is the use of a comma to join two independent clauses. For example: It is nearly half past five, we cannot reach town before dark. Although acceptable in some languages and compulsory in others (e.g. Bulgarian or French), comma splices are usually considered style errors in English.