My opinion: Ich is like intestinal flora. It's always there but if you stay healthy, it'll never bite you in the ass.
Unless you take antibiotics one day and throw off the balance and end up at the doctor with some random bacterial imbalance aka stomach flu
I know for a fact there has been Ich in my tank but I dose with Selcon and Garlic, don't stress the fish out and have stable water conditions. So it never rears it's head. (Unless it's now past the 10th generation lol).