It’s not….it’s a natural pathogen that lives in our mucous membranes. Majority of people are asymptomatic carriers. When there is an episode of immuno compromise, such as a mild cold or otherwise, the bacteria has the opportunity to multiply and cause disease. Same goes for meningitis… bacteria need a method of entry or a compromised immune system to cause disease. Plus, unlike viruses, they need intimate contact (into mucous membranes or through saliva,etc) to cause disease.Streptococcal bacteria is not contagious? Since when?
The reason I bring this up is the theory of bacteria causing disease in a fully immuno competent host with no evidence of tissue damage is highly unlikely. There has to be some sort of stress to the tissue or the immune system for it to cause disease.
I believe that the bacteria these corals are exposed to in nature is different than the ones in our tanks and they are exposed to foreign bacteria that they don’t have immunity against. Add to that a small injury or the stress of handling, shipping etc, and bam… BJD.