[Edi]

There are two sorts of disease bringers: Reaper, which is what Harvester of Sorrows, Poison Golem, Pazuzu and Mabakiel are, and Plague Carrier (afflicts units in the same province with disease but does not kill population), which is what Disease Demon, Apparition and all forms of the Eater of the Dead are. What you are talking about is probably disease cloud, like hydra has a poison cloud. Disease cloud has no icon.