Easy as pie achievement! [Edit] This achievement must be done on Veteran, which was not the case when I originally posted the solution or got my achievment. Thanks to those in the comments for pointing that out.
All you need to do is pick up 2 items and give them to Jorvuld at the end.
After you beat Matriarch Aldis (the third boss - giant on the ice platform) you will see a corrupted leimenid corpse in the water next to the rock face. Interact with it and choose option:
"Take sample of diseased corpse for later research." - The whole group gets it.
Now, after you beat Plague Concoctor Mortieu go over to where Jorvuld is standing and interact with the alchemy station behind him and choose the following option:
"Take sample of disease concentrate for later research." - The whole group gets it.
Final step: After the final boss fight, simply wait for the green ghost to fly into Jorvuld and when you interact with him, choose the dialogue option to cure him.
Achievement is yours!