Myco Ive noticed, is more so flared up when cage is dirty and cage not cleaned enough to disinfect the germs. Cant be passed on if one has it but if caught late the rat will go down hill in matter of days losing weight if you dont have baytril to give.
Now Pneumonia is another thing to watch out, can look like myco flare but its not. Wet Noses is a bad sign and can be passed onto the rest of rats in the tub.
I also hold back females and males when needing to replace groups. havent had to add any rats for a long time.