[Taqwus]

HJ --
Plague spells get announced, although they look like the ordinary random events if memory serves.
Bane venom charms, Harbinger of Sorrows, et al, result in something like "A deadly disease is spreading in...".
I don't recall sites giving the same message, however. Perhaps you ran into a Chillsick Swamp (IIRC; forget whether death or nature) or some other disease-causing nature site.
The 'diseased' affliction generally doesn't spread, except from the BVC, and Woundflame (?), and similar items, or from inherently infectious units (Harbinger of Sorrows, Mummy come to mind).
If memory serves, disease can come from ordinary wounds just as with any other affliction.