Miasma (μίασμα) means “stain,” “defilement,” or “the stain of guilt” in Greek. It is usually translated as “pollution” in English, although there is no concept in English that precisely corresponds to miasma. Miasma is a god-sent disease that is caused by a murder that has not been atoned for (with proper purification rituals). A miasma can fall upon an entire city when one man in that city is guilty of a murder and has not atoned for it. A miasma can infect everyone on board a ship if one man on that ship is guilty of murder.
Miasma can spread like a disease, and it seems to be the objectification of guilt. In other words, the guilt is understood as a kind of disease that can spread to everyone who comes in contact with the guilty person. The concept of germs was not developed until the 1880s, but the effects of contagion were clearly visible from a very early time. The Greeks seem to have believed that the cause of contagious disease was guilt for an unexpiated crime. The only way to stop the spread of the disease was to find the guilty party, have that person atone for their crime by paying a penalty (usually a fine or banishment), and undergo an expiation ritual (frequently a sacrifice of a suckling pig). You could classify miasma as a psychological myth (since it is the manifestation of the emotion of guilt) or you could understand it as a natural aetiological myth, since it explains the natural phenomenon of contagious disease.
When Oedipus the King begins, the city of Thebes is infected with a miasma: a disease has fallen on the crops, the cattle are dying, a plague is raging through the land, and all the children are stillborn. The Oracle at Delphi proclaims that the miasma is caused by the unexpiated murder of Laius, the previous king. Apollo declares that the murderer is still living in Thebes and that he must be found and punished (either by banishment or death) in order for the miasma to come to an end.