Diana Walstad (written 2009 and updated 2013) 
MB (mycobacteriosis) is the number one chronic disease in aquarium fish. It is responsible for about half of fish deaths due to unknown causes. Because MB often has no obvious or defining symptoms, hobbyists underestimate its prevalence. If a newly purchased fish stops eating and dies after a few weeks, most hobbyists do not suspect MB (much less know what it is). Additionally, chronic MB weakens the fish’s immune system making infected fish highly vulnerable to other diseases. I wonder how many hobbyists have attributed their fish’s death to other pathogens, when the underlying problem was chronic MB? How many hobbyists have attributed their fish’s death to old age or inbreeding, when the real problem was MB?
No fish is safe from this scourge. MB outbreaks have been reported in scientific laboratories, zoos, commercial fish farms, natural ponds, oceans, etc. Since the disease is incurable, the consequences to fish breeding can be devastating. As a fishkeeper, my own experience with MB was most unpleasant. Fortunately, there was a satisfactory outcome.