There are certain types of mites that suck the blood on a chicken & eventually they will become unthrifty & die. You can Google chicken mites & see what kind they are. Birds are subject to all kinds of parasites both internal & external. They also are subject to quite a few diseases, some being brought in by wild birds.
I have never had a lot of luck with the Marans breed for another that seems to be a weaker breed. I don't know if it was just the birds I got, but they were from a breeder, not hatchery. I have had problems with them having heart issues & just getting sick easier than the other breeds. I do have two hens left in my laying flock that seem to be doing fine, so maybe they are the exceptions. I gave up on trying to raise them so now just use the two remaining ones for the dark eggs. I have found that I like the Welsummers better, they're hardier birds & lay pretty speckled dark eggs. They're very good foragers as well for my free range flock.
I looked up Lymphoid Leukosis of chickens Danz & that liver looks grizzly, yuk!