Exactly, ich is a parasite, not a virus or a bacteria.
Fish bring it in from the wild. Due to the nature of the parasite's lifecycle, fish who have ich in the wild will not likely have it for long as the parasite just can't keep up with fish movement all over the reef. However, in the captive confines of an aquarium, ich have access to free food [fish] and nothing will stop them from proliferating except treatment.
Even if a fish does not have ich when caught, if they are placed in an infected tank, whether that be at the wholesale level, the retail level or your own tank, then they become infected and carry it with them wherever they go [until they and the tank they have been in are treated]. That is why ich is so prevelent in the hobby and the infection level so high.
That is why it is so very important to set up a QT in your home so you be sure that an infected fish does not enter your own display. You might get a fish that looks perfectly fine. Then bring it home and introduce it directly into your display. 3 days later all your fish have ich. The ich is not only on this fish, but in the water that the fish was bagged in. What do most hobbyists do when they get a new fish. Float the fish in the bag in their displays. Then release the fish and the water into their tank. If fish or water has the parasite, then you have just introduced the problem to your display.