Yes, ich is a fish parasite. Just like dogs have flees, fish have ich. In the wild a healthy fish might pick up a few parasites, but because the fish are always on the go, once they loose the parasites they caught, they likely will take a breather from them before running across them again. In the captive environment [fish tank] fish have no choice but to be food for for ich. They can't swim away from it. Thus, in a fish tank, ich can quickly reach plague proportions. There is a post in the FAQ Thread that details the lifecycle of this parasite that will help you understand. Understanding is half the battle in turning this dreaded hobby problem into a minor nuisance.