My wife and I just went through a pretty bad bout with ick. Even with the research I had done on it in the past and how to treat it, we still lost over 120.00 worth of fish.
The bottom line is that nothing CAUSES ick...it is either in the tank or it isnt. Stress will not give a fish ick. Temperature will not give a fish ick. Shock will not give a fish ick. These may lower its immunity and give the parasite the oportunity to attatch to the host, but a fish cannot contract a disease or parasite that is not already present in the water. You would not tell your doctor "the temperature in the house dropped to 50 last night and I caught malaria." Why...Because we know temperature does not cause the disease and we know that it must be present in some form for it to be contrated in the first place. Same with fish.......
There is an old myth that ick is always present in the tank. This CAN be true..but isnt always. If the first fish you bring into the tank has ick, and you never fallow the tank, then yes....ick will always be present. But, if you take steps to ensure that ick never makes it into the tank OR wipe it out if ick is found to be present...then the myth is false. The life cycle from cyst to visible parasite reproduction is about 4 weeks. It must have a host to survive. Fallow the tank (meaning fishless) and QT
any and ALL current and future fish for 5-6weeks and you can be sure that the parasite is no longer present in the tank.
As always...I have an article on the subject if anyone would like to read it.