Build: v2.4.0
19051.2806817
At the moment, Ku'Gath's Fever symptom only grants +150 Favour when spreading a plague to settlements, despite the UI stating this effect applies to ALL armies and settlements. If the favor is to be granted "when a plague is spread", it should behave as the Rot plague with summonable Nurgling units (the resource is granted whenever an army or garrison is infected).

TO RECREATE:
Play Kugath, infect Kugath or any other lord with a plague with the Fever symptom, then transfer a unit from the infected army to another army. Your faction's favour will not increase. If you instead send a plague cultist to a settlement, you will see favour go up.
To fix this, I went into the faction_set_effect_bundles table in the database and changed two cells to the values shown in the images below.
Before fix:

After fix:

After changing these two cells, Fever granted the 150 favour when spreading the plague to an army. I'm guessing this was an error copy/pasting values when the table was created.