Years ago we had about 35 chickens. I went out to do the feeding and gathering and found almost half of my flock dead. One was partially pulled through the fence and the others had a hole in their heads. They wheren't even eaten. The one that some thing tried to pull through the fence did so piece by piece.It was just a slaughter. DH was on the road at the time. I looked for anywyere something could get in at and patched it up but the run wasn't covered and the door to the part of the barn that the chickens occupied was open. I wasn't very chicken smart at that time in my life. I didn't think what ever it was, was climbing the fence. I thought more of digging under or finding a hole. Next day more chickens gone but not as many. DH was home that night and spent the night with the chickens and a shotgun. He caught our murderer, it was one of the biggest coons i've ever seen. So we thought it was all over.
I was wrong. We lost them all the next time. Another masacre. I gave up on chickens for many years. Now, when we first got chickens again a few years ago we lost about 6 to neighbors dogs. Havne't lost anymore as of yet to predators. None to sickness.
We have a coop make out of old barn wood for the frame and tin around the bottom and top and the roof and wire around the middle of 3 sides. The door is a storm door for a house that is secured with a hasp and lock. We have a covered run securly attached to the coop. The only way in is to have a pair of wire cutters or a pick and shovel. The ground it sits on is mostly rock under a layer of dirt and grass. Only a snake, mole or mouse could get in and the rooster would make a short snack out of them.