What is the reason for the plague in Thebes in Sophocles' Oedipus Rex?

At the beginning of Sophocles' Oedipus Rex, there is sterility and a plague at Thebes. This is presented to the audience through the dialogue between Oedipus, the king, and the Priest (1–67). Oedipus has already sent his brother-in-law, Creon, to the Oracle of Delphi in order to ask Apollo why this is the case. According to the Oracle, Apollo regards religious or moral pollution (miasma) resulting from the murder of the former king, Laius, to be the cause of the plague and that the cause of it (i.e. Laius' murderer) must be killed or expelled from Theban territory. Later in the play (300), Oedipus summons the blind prophet Tiresias so that he may discover the murderer.  Tiresias tragically reveals to Oedipus that the king himself is the cause (since he had killed King Laius). Oedipus doesn't believe him — since he did not know who Laius was when he killed him — and sends him away.

So the tragic truth is that Oedipus, who unwittingly killed his father, the previous king, is the cause of the Theban plague.