Thats a tuffy, LW. The health of the bowel~or the lack thereof, IMO, is the likely cause of gleet, though everything I've read on the subject says there is no absolute known cause. Here's as plain as I can state it:
We all have good and bad bacteria/yeast in our bowels. The goodies keep the baddies in balance. When you have excessive loose stool/diarrhea, these all can be stripped out, creating an intestine that doesn't have the needed goodies/baddies to digest food properly. Unfortunately, when they start to repopulate the bowel, the baddies have a quicker growth pattern, especially if there are no goodies to keep them in check.
The baddies cause a lot of problems when they are allowed to run rampant. They cause even more diarrhea, ulceration(holes) of the intestinal walls, infection of the skin around the vent(gleet), etc. When they get that bad, often it will take a special antibiotic to kill the baddies enough to let the goodies grow properly. These antibiotics are expensive and very harsh~sometimes they just create a baddie that is immune to their charms. Unfortunately, the antibiotics also kill any of the goodies that may have been trying to grow. It is advisable to follow the antibiotic usage with acidophilus, brewer's yeast, lactobacillus, yogurt, buttermilk, cottage cheese, etc.
The infection is said to be contagious but it really would only affect other mammals with imbalanced bacteria in their bowels(already in a weakened state and open to infection) or if the coop environment was such that it encouraged the growth of the baddies~damp, too acidic, no air flow, overcrowded.
See the importance of creating a balance in the animal and the environment to prevent the spread of contagion? Try to promote the growth of the goodies because they are valuable.
You can either bleach everything in sight and hope for the best and also kill off any affected animals~or~you can insure that, if something does affect one animal, that it cannot affect the others because they are too healthy, the coop environment is too healthy to support the baddie growth cycle, etc.
Keep in mind that when you bleach everything, the first thing that grows back...is the baddies..with a vengeance. You can't fix things with applications of disinfectant or dosing with meds or ACV....these might help temporarily but they are not a fix.
ETA: No PhD,here...just 20 years in a medical profession. Ya kinda get real familiar with the bowels in my field.