Your fish dont really get ich from other fish either. Ich is ALWAYS in your tank, everyones tank for that matter. Ich, like most parasitic organisms has different life stages, and only part of its stage is actually on the fish. The other is in the water and gravel and stuff. When a fish is healthy its own immune system is strong enough to keep the ich from attacking. However, if your fish is stressed from say, bad water, an aggressive tank, stress of shipping, or another disease the fish's immune system will be down making it more apt to get ich. Feeder fish can be full of nasties so its possible your fish came down from somthing else from the feeder and then it got the ich.
Since the ich has different life stages increasing your water temp to about 85 with some salt speeds up the ich's development so it tends to leave your fish or "heal" faster in a sense.
Well at least this is what I have read anyway. lol