When tieresias says "the city is sick" he means pestilence is spreading because

Tiresias (in Greek mythology) was the prophet of Apollo in Thebes, and the reference you're making is from Sophocles' play, "Antigone."  In the play, Antigone's uncle, Creon, has inherited the throne of Thebes.  Antigone's brothers, Eteocles and Polyneices, are dead.  Creon had issued an edict prohibiting a proper burial for Polyneices, because he was seen by Creon as a traitor.  But Antigone ignored her uncle's law and buried her brother.  She was caught, and Creon then declared that she be buried alive.  That's when the gods sent a message through Tiresias, saying that it was Creon's fault that the city was sick with plague. The gods were displeased with him.  

There's lots more tragic events in Sophocles' "Antigone."  (It is a Greek tragedy, after all.)  But I think we've answered the main question you had!

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