It can be introduced on shop bought fish i agree, but what that article and other more in depth articles are saying is that it is in a form of suspended animation in any aquaria, it is just waiting for the right circumstances to start it's lifecycle.
I can't see the reference to "that article" earlier in this thread...is there an article somewhere about ich?
I agree with the many authorities that ich is indeed present in many home aquaria. There can be no other explanation for a situation where nothing is added to an established aquarium for months, even years, but a sudden stress event (such as a heater failure, chilling the tank) results in an outbreak of ich. Obviously it had to be there, unobserved.
The explanation I understand is that just one or two tomants need to survive to maintain the parasite's presence. As tomites first attack fish in the gills, we won't see them. An occasional flash of one fish may indicate this, or be something else. The "immunity" the fish build up is also involved, in the form of some thickening of the protective slime layer. All of this makes sense, when one considers that wild fish never die off in great numbers from ich, and it is not an aquarium-confined parasite though within the confines of any aquarium it will be more evident than in the habitat.
Bob Fenner on wetwebmedia writes:
Ich is almost always present in freshwater systems and is parasitic on most if not all freshwater fishes. All that it takes to become pathogenic (actively infectious, disease-causing) is a strong strain of ich (e.g. an import from a newly added specimen), a not-so healthy, poorly-resistant host and/or a poor environment for the fishes. Re the last: Note that all diseases are to degrees environmentally linked. If the fishes are initially in good health, put into a suitable, stable home, the chance of outbreak is small.
Stress is the direct cause of about 95% of all fish disease. Obviously the pathogen must be present/introduced, but in 95% of the time the fish will manage to avoid contamination by some means. It is only when they are stressed that this fails, which is why every single factor that can cause stress must be avoided as much as possible.