There are times when immunity might drop just briefly for unknown reasons or for example, when it happened to one of my snakes, it happened after she got gravid. Can also happen if the immune system is fighting off something that is asymptomatic such as a virus. Once the bacteria get the upper hand, that's it. Doesn't matter if the snake is otherwise healthy, they're going to need some help to tip the balance back where it should be and that's what baytril does. It doesn't kill the bacteria at all. It just keeps it from reproducing at a normal rate, if at all. That allows the snakes immune system to catch up and kick the infection. I'm sure this would do something similar. Keeps the numbers way down, tipping the balance in favor of the snake.

Most cases of RI are just common bacteria that normally wouldn't harm the snake. It's just when something stresses the immune system, the bacteria get the upper hand. In my case, the RI wasn't contagious at all. It was just common bacteria. Completely harmless to a healthy immune system. She just needed the baytril to give her the upper hand on it again. She went on to live just fine and didn't get another RI and none of the other snakes that were housed with her got sick.

I went through something similar myself. I had just gotten over a long and nasty cold and was feeling better. Then all the sudden (like overnight) I came down with pneumonia, was coughing up blood , 104.7 F fever, and I almost died. The bacteria responsible was completely harmless under normal circumstances, and it wasn't contagious. It just got the upper hand because I was fighting off that cold virus.