I mentioned about a week ago that I thought this was the case, but I've had time to create a fairly rigorous, repeatable test for it. To definitavely rule out all other causes, the UB needs to have the Plague and Ooze abilities, and not be wearing any armor with a Thorns attribute. staying well away from towers (so that creeps don't become infected as they walk past), find a group of creeps coming towards you. First with ooze off, walk through the creeps, using a force move (as opposed to attack move) so that the UB doesn't make any attacks. Although he may take a little damage, no enemy creeps get the green plague letters above their heads. Once past all of the group of creeps, turn plague on and walk back through, again not attacking anyone. This time, once sludge has hit the enemy creeps once or twice, suddenly the whole group will get plagued. As the only difference (and source of the UB's damage) between the two groups under otherwise identical circumstances is ooze, I think it's confirmed that ooze damage causes plague. Considering that this means any enemy units anywhere near the UB regardless of their interactions (or lack thereof) will automatically get the disease the moment the ability goes active, I think it's something that needs to be addressed.