Kill Morphaz in the sunken temple of Atal'Hakkar, and return his blood to Greta Mosshoof in Felwood. The entrance to the sunken temple can be found in the Swamp of Sorrows.
Now the last part, the second cure.
The green drake Morphaz is known to be immune to all forms of poison and disease -- many druids died to bring us this information. There is no doubt his blood is potent for our needs.
A courier from the Cenarion Circle will take the parts you have already collected while you gather the last. Summon your allies and seek out the drake's lair in the sunken temple of Atal'Hakkar. Bring his blood to Greta Mosshoof in Felwood.