Yes, nearly all chickens are exposed to mareks. It is a type of chicken herpes. Thee is a primary infection that most chickens do not get very sick from Some ill though. Then a secondary cancer that hits a percentage of chickens after two years of age.
It is similar to the one in humans that we...
That could be mareks cancer. About 5% of all hens get it.
The necropsy is a very good idea but be wary of advice if it is mareks. Places like that freak out about mareks and give questionable advice about what to do with your flock.