Wally, doesn't really make a difference if a fish is health or stressed, if Ich is present in a tank, or is introduced, any fish in that tank can get it, regardless of it's condition. As a matter of fact you see this all the time. Person has a tank loaded with healthy fish, then introduces a new fish, which has Ich, then proceeds to lose all of his healthy fish from the Ich. As for seeing an eel, or even a Mandarin, get Ich, well I've seen both. It's rare but it does happen. The Ich, white spot, actually was on the fish eyes and not their body. Also, having a thick slime coating doesn't prevent the Ich from entering the fish mouth, eyes or even gill areas. What the heavy slime coating does is make it harder for the Ich, white spots, to come out through the fish skin. Usually a fish will produce a lot of extra slime when they're affective by Ich too, but that doesn't stop them from continuing to get re-infected again, especially in a close system like ours is.
Yes, it's ignorance that kills so many fish, as people don't take action when Ich shows up in their tanks, or just keeps adding more fish when the old ones die, not taking into account that Ich is still present in the tank. But the thinking that my fish has stress, and because of that my fish just got Ich, is completely wrong and is a very common thought process, and not a good one. People not taking into account that Ich is a parasite and a fish having stress isn't the reason that it came down with Ich just leads to more fish dying.