Yeah, all domestic rats have myco, usually they only get active infections when sick with something else or otherwise stressed. It's an odd thing, though, because it can progress to the point that it kills them, it just usually doesn't. I had a rat who died of myco, I think -- she had the hallmark coarse fur, lethargy, and wheezing. Those little guys really are a fucking thrill a minute, especially their tiny little lungs. Damn them for being so fragile!