IMO, HLLE is not contagious; however, if all fishes are exposed to the same aquarium conditions (diet, water quality, etc) then they may all show the same symptoms. Bacterial infections are caused from poor water quality. Several years ago, I had a fish that received a very, large and deep gash from a yellow tang; to my surprise, it healed up with no complications. Parasites are different all together, their life cycle allows for different types of transmission. In the early stages, they are attached to substrate, rocks, snail shells, etc; then later they are free-floating and can attach easily to fish. I once introduced ich into my tank via a large snail purchase
HTH