In my time keeping fish, every single outbreak of Ich has been easy to trace - it came in on store fish. It can take a little time to run wild, but the source was always very affected by it. Once I learned to spot it early, it ceased to be a problem.
A friend got some shots of Ich in the Amazon. Each affected tetra had one or two spots. It appears to be a basic, low grade parasite that wild fish can swim away from. The parasites get lost in the massive volume of moving water, and just get enough hosts to keep going. Fish crowded into tanks can't outswim the Ich that bursts out of the white cyst, and we get outbreaks the way crowded human cities also have trapped and crowded epidemics. In the wild, it looks like it's an itchy annoyance that a fish puts up with for a week.