Okay, did alot of reading, and think I answered my question. Seems that (AM) is very common in small flocks , but rare or none in production houses due to sanitation and short lives of the pullets after they lay for a year+.
It is very common, can come from anywhere, wild birds, dirt, etc. Spread fecal-oral, (you know chickens they walk in their own poop).
The most common symptom is to have normal looking chickens, then one dies, and picking it up you notice that there is no fat left on it's body. They just wasted away over months, and it was never noted. There are sometimes other symptoms.
There is no cure, no medicine, no vaccine for it. It's difficult to diagnose prior to death. After death if you do a necropsy, you might find the nodules covering organs. For us back-yarders, it's very hard to prevent as well.
This looks like lengthy reading, but is more thorough than other things I've read
http://www.oie.int/doc/ged/D9352.PDF