Ichthyophthirius multifiliis = ich.
Ich is a little parasite that floats around in the water, and at one stage in its life, will attach to fish if their immune systems are compromised.
Ich occurs in all natural freshwater as far as I know. There is also a saltwater form, but it is a different species.
If fish in the wild didn't get ich, there probably wouldn't be ich in aquariums. When you get fish from the store, the ich comes with them, along with a lot of other parasites and bacteria. As long as your fish are healthy, they shouldn't get ich.
Graeme