If you don't take good care of your fishes and even your pond as well, it would make the possibility of your fishes getting sick higher. If you don't feed them well, cannibalism will start and that's not a good sight.
The risk of disease is high when the water quality is poor or when there is overcrowding in the tank. Inappropriate diet is also yet another reason for sickness in fish. Bacteria and fungi also cause disease in fishes.
All fish are susceptible. This is why keeping your tank water clean and balanced is important. It not only prevents disease but it keeps it from spreading if it does arise from a sick fish or contaminated plant.
Fishes are susceptible to illness, but few fishes are easy to maintain. You can choose these fishes in the beginning and from there you can choose whichever fish you like. I love Flower Horn fish, it's easy to maintain and looks super cute. It's carnivorous, you can feed them dried shrimps, special fish food, or live fish it's up to you. It fits perfectly in medium-sized tanks.