aside from being passed along from other [fish, decor, whatever], it is also believed that the little ich-causing organism is present in almost all situations, but that fish have built up an immunity to this very small amount that is usually there. therefore, stress due to any number of reasons [poor water conditions, temp. change, etc.] can bring down a fish's immune system [stress: #1 killer of fish] and allow the ich to invade and spread and grow.
and just an FYI, some people will disagree with this, but [in my opinion] the very VERY best treatment for ich is to prevent it from happening in the first place [that's not the part that people will disagree with]. i ALWAYS keep 1 tbsp. of salt per gallon of water in my tank as a general tonic and infection preventer [please don't try to argue with me, i will not be around to see it and besides, salt is something that is commonly used to kill infections and such.. ever had a cancor sore? yeah, it can smother infectious bacteria and stuff] and keep my tank temp. at 78 degrees [and make sure all my fish are compatible with this temp obviously]. the combinations of the CORRECT amount of salt and ~78-80° temp helps to keep the fish stress-free and the bacteria and other small baddies at bay. and i've used 3 tbsp. salt/gallon plus ~80-82° temp to kill off ich successfully [not in any my tanks, i've never had ich in them, but in my mother's. i've also used this combo. to rid my GSP of a fungus he had that popped up on his eye]
oh, and sorry for the mid-sentence ramblings