Maybe it's just always in your tank but healthy fish keep it in remission and over the course of months it dies out. There have been strong reports that it takes 11 months to eradicate ich when no outbreaks are present or no new stains of it are introduced.
Exactly Kevin. Also supplementing food with amino acids will boast immune systems. If fish are showing signs also add metro and focus medicines to food for 30 days. This makes the cysts fall off and not reattach because the ich does not like the medicine that is in the fish.
selcon is good stuff. you can also add some of it to some rods food for a good immune boost.
Dont mix it with water (I don't ever mix with water). I just break a chunk of rods off, add your additives (aminos, selcon, etc...) let it defrost in a cup/bowl. Mix it all together. Let it sit for a few minutes to absorb. Then just dump it in, or spoon feed it to the tank. Fish will then eat big chunks of the food, instead of just pieces.