At Weathertop, during his journey to Rivendell with the One Ring, the Hobbit Frodo Baggins was stabbed by the leader of the Nazgûl. A fragment of the blade remained within the wound, working its way toward his heart and threatening to turn Frodo into a wraith. Elrond was able to remove the shard and heal the wound, but each year on the anniversary of his stabbing Frodo became seriously ill. Only his eventual departure to Eldamar afforded a permanent cure.
Athelas (or Kingsfoil) is known to slow the poisonous effect of the morgul-blade. This remedy is also known to heal other Mordor illnesses.
Another victim of a morgul blade was Boromir, the eleventh Steward of Gondor (not the Boromir of the Frodo's company). He eventually died of his wound but did not become a wraith.