I found this,
Algone.com - Ich and your aquarium fish, on itch. According to the information provided, the parasites that cause itch will always be present in an aquarium once they are introduced. As long as the fishes immune system is not compromised in any way, they can keep the parasite at bay. Once the immune system is weaked due to stress (from bully tankmates, bad water quality, newly introduced to the tank, etc...) is when the parasite is able to attack the fish and cause outbreaks once again. This is the main reason many people promote the idea of a strict quarantine period before the introduction of any new tankmates. If you don't introduce it to your tank, you will have less of a chance of having problems with it in the future. Hope this helps!