Fish can most certainly fight off ich under the right circumstances. Stress is what lowers their immune systems and then you see the parasite take hold. I have never QT'd any fish and many of them had ich when they were introduced, and all fish made full recoveries, except for one Achilles tang. When I tell you I had a lot of fish in the years I've been doing this, I'm talking hundreds. These fish were sold on a regular basis, so I didn't keep them for very long, but I put them in a system that was connected to my main display.
Changing tanks/coming from the LFS
to your tank, ammonia levels/poor parameters, bullying, and undersized tanks are mainly what starts it. Without these factors present, you won't have a problem with ich, even if it's in your display.