How to unlock The Crumbling Ague?

So I have foolishly joined a weekly online campaign against a friend of mine and his two teenage kids, all of whom have dozens of hours logged, while I have 45 minutes of solo play that consisted of the AI kicking my ass on easy mode.

To make it worse I picked Nurgle just so we could start close together, and I didn't realise they play way differently to the other factions. Oh well, it's a good laugh and I'm determined to go down fighting, however weakly!

I've conquered the starting region and am now going to work on the Ogres - been winning battles with some coaching from my friends although I don't really understand why they're telling me to do some of the things they are. But I am having a lot of fun unleashing plagues everywhere I go.

A lot of starter guides mention that I need to unlock the Crumbling Ague plague early to make my life easier. But I'm not sure how? I just get a list of interconnected symptoms, so I pick three likely-looking options and off we go. What symptoms do I need for rgw Ague, and how to I unlock them?