wild fish have some tolerance / immunity to ich, otherwise ich would not exist, a disease that kills too many of its hosts looses its ability to propagate and maintain a viable population. chances are you introduced the ich with some new addition, probably chromies. I would at least treat them in a separate tank and keep the main tank fallow for a while or the issue will arise again, probably sooner than later.
ich in itself, with the exception of the extremely bad cases, does not kill in itself, it either causes the fishes immune system to go haywire or collapse from the infections in the attachment spots or causes enough scar tissues in the gills to cause death by oxygen starvation. if the fish has enough immunity to ich than you will see no sign unless you dissect the gills and look at them with a microscope, the fish will just be an active carrier that suffers little from the infoection but will cause outbreaks in other individuals.
ich in itself, with the exception of the extremely bad cases, does not kill in itself, it either causes the fishes immune system to go haywire or collapse from the infections in the attachment spots or causes enough scar tissues in the gills to cause death by oxygen starvation. if the fish has enough immunity to ich than you will see no sign unless you dissect the gills and look at them with a microscope, the fish will just be an active carrier that suffers little from the infoection but will cause outbreaks in other individuals.