In short, the people of Thebes are in a pretty bad way at the beginning of the trilogy. The Priest of Zeus tells Oedipus, their king,
For, as thou seest thyself, our ship of State,
Sore buffeted, can no more lift her head,
Foundered beneath a weltering surge of blood.
A blight is on our harvest in the ear,
A blight upon the grazing flocks and herds,
A blight on wives in travail; and withal
Armed with his blazing torch the God of Plague
Hath swooped upon our city emptying
The house of Cadmus, and the murky realm
Of Pluto is full fed with groans and tears.
In other words, the priest tells Zeus that Thebans are in such poor shape that their problems threaten to overwhelm the entire country. Some disease has struck their crops so that the corn will not grow; a disease has struck the herds of animals, like cows and sheep, that provide food and milk; finally, a disease has struck women who are pregnant and in childbirth (or soon will be). The priest fears that Apollo has afflicted their city with all of these terrible diseases that threaten Thebans' livelihoods and is filling up the Underworld with the shades of the multitudinous dead.