I believe most RIs in rats are just flare-ups of myco. They already have the myco, which I guess is usually sort of dormant. But something such as the stress of moving to a new home, dusty bedding, or any number of other things makes it flare up. If a rat/mouse has a strong immune system, and the living conditions, diet, etc are good, the flare-ups should be relatively mild and they should recover. On the other hand, it can be fatal, as you have seen.
I had a similar issue when I was starting up my mouse colony. I went with the theory that if their immune system was strong enough for them to recover from the myco flare-up, then there was nothing wrong with having them in my colony. I gave them echinacea to boost their immune system. After I culled the ones that didn't do so well, I ended up with about half of the mice I had originally purchased.
Here is a link to the thread I made at the time. You might find some of the info and comments in there to be useful:
http://www.ball-pythons.net/forums/s...ad.php?t=73792
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