HITH is pretty much always capable of becoming an issue. Some fish are much more suseptible to it than others. Catfish, oscars, pike cichlilds, and some say angel fish (though that hasn't been my experience) are common ones. It isn't really contagious, unless the water is poor quality. It is easily treatable in most cases. Regular weekly water changes, a GOOD filter system, and some ocean salt used at 1tbsp/10 gallons every water change clears it usually within a couple weeks. I had a pike cichlid a couple years ago that had very bad HITH and that's how I treated it. He had bad scaring from it, but was very healty after words, and was still a beautiful fish.