Human TB is a crowding disease. It spreads easily in prisons, for example. Fish farms create those conditions for the fish version of the bacterial disease, often caused by Mycobacter marinum. Pretty well every species group has its own Mycobacter, as it's a tough and opportunistic family.
It's a sneaky one. The fish catches it, and encysts it internally. That's a cause of crooked spines and such, a symptom. It depends on where the cyst is. As it advances (and it can sit encysted for a long time, so a fish can be a carrier for many months to years) it often forms a rectangular plaque type sore on the flanks of the fish. That's a rainbowfish special, as they are very often hit with it.
In most cases, it radically shortens the life of the victim. A fish that might live 12 years gets 3 or 4. But as infected fish die, the bacteria break out of the cysts, and can explode into an epidemic and take out a lot of fish. Never leave a dying fish to die in the home tank. Never buy a deformed or unhealthy looking fish out of pity. "Rescue Bettas" are time bombs.
I caught this disease through a cut. It was cured with an antibiotic cocktail taken daily for six months. This wasn't fun. I saw a rectangular plaque on the back of my hand, and it looked like what I had seen on fish. A second appeared on my wrist, and off to a dermatologist I went. It didn't hurt, and the plaques never became sores as I acted promptly. I still have a scar from the biopsy, but that's all. If I am given a test for tb, I will test positive for antibodies, forever.
My doctor taught dermatology at a major university medical school, and mine was the first case he'd seen. I was very popular with his students. Transfer to humans is rare and tends to need special circumstances. I had 2 jobs, a family and was caring for a dying family member. I was exhausted, and therefore vulnerable. If I encountered it now, I doubt I would catch it. The only other person I know who caught it was an alcoholic, and was in rough shape. Generally, you can fight it off easily if you are healthy. If you are at all immune compromised, be careful.
The bacteria can't live in our bodies. We're too warm. It hangs on on the surface, in our lymphatic system just below the skin, where we are cooler. It travels slowly as it colonizes us. It is a serious annoyance - no more than that.
Gloves. You need them up to your elbow. I avoid water changes if I have fresh cuts, but my main response has been to proceed as if every farmed fish has it dormant. I wash my hands after work in the tanks, and let cuts heal without wetting them. I don't use gloves myself.
It can be from wild caught fish (there's a thread right now) but that's rare. I isolate my wild caught and farmed fish for at least a generation. I don't combine them. I've never seen a wild caught fish show any symptoms here, unless I put a farmed fish in the tank. Once I learned that, it has been confined to some farmed fish only. I no longer keep rainbowfish, sadly.
