Chickens not laying

I have around 18 hens that are 18 months or older. Earlier this summer I was given 4 chickens that had stopped laying for unknown reasons. Three of them (3 years old) were put in the stewpot after a couple weeks but the 4th one I promised to let her live out her days. Now my hens have almost stopped laying (only 4-5 eggs per day). I know that the shorter days affects them & I have a light on in the coop to give them the extra light. They are also just getting done with their molting so I am expecting fewer eggs until they are completely done. I have a concern though because the hen I promised to keep died last night for no apparent reason. She seemed to be perfectly healthy though I have no idea how old she was. Possibly 3 years old like the other hens that were given to me. My concern is this: is there an ailment that would cause a seemingly healthy chicken to stop laying eggs and if so can it be spread to other chickens? Any suggestions will help. Thanks

 

I shut up the four chickens for a couple weeks when they came in and all but the one I kept were butchered within 2 weeks after getting them. I don't think it would have made a difference anyway because it was over 4 months ago that I got them. Even if I had quarantined them for 40 days the one would have still infected the others if she had something contagious. Up until last night the one that died was active and appeared healthy. When I went down to shut them in for the night she was on the floor instead of on the roost and this morning she was dead. There really wasn't any sign that she was sick. It may just be that she was older than I thought.

All the chickens appear healthy. Their eyes are clear, no runny nose, feathers look healthy & poop looks normal. Their appetite is good and they are very active. A couple of them do have some minor drippings stuck on their butts but that could be from the veggie scraps I have been feeding them or the change in their feed. I have been feeding them flock raiser because I also have a turkey in with the chickens. I am butchering the last turkey this weekend & in the process of changing the chickens back over to layer pellets. We had a couple bouts with coccidiosis this past year, once in the spring & then again this fall but that's all cleared up now.