Well - yes and no, IMO.
Ick is a pretty common issue for fish but healthy non stressed fish can fight it off. Stressed fish, such as those in transport or being added to a new tank come down with it, just like we can pick up a cold after a period of intense stress. Normally we fight it off and aren't always sick. Same for these fish. The bugs that cause these diseases are basically always around to some degree - but the weakened among us, whether human or fish, become ill.
But it is yet another stress on an already weakened fish, so it can contribute to death. Some will survive it, some won't. That is true. It is similar, but not the same bug as freshwater Ick. For this reason the saltwater version is more appropriately abbreviated as "Crypt."
