Yes you want to treat any fish that has been in the tank or has come in contact with the fish from the infected tank.
Chaces of the fish or a fish dieing are slim AS LONG as you treat the tank. Once they develope a bad case it will attach to there eyes and gills. Once the parasite has attached to the gills the fish start to experience breathing troubles and will swim around the tank as if they are being burned.
this is the parasite beneth the skin.
So yes, fish left untreated can die from ich. I would suggest treating them all and NOT itroducing new fish till the problem is cleared up. If you continue adding fish the parsite will continue to thrive. And as I mentioned in most cases evrytime the outbreak accurs it can and will be worse.
danny