They could have worms and you'd never know it. This is why I recommend a worming program set up at your convenience. When you see them in poop, it only means they have an infestation and there's no more room inside the chicken for them to go.... but out. The worm(s) excreted will die, they cannot survive outside the host. Each worm sheds thousands of eggs to be deposited on the soil. Usually, if a chicken is acting lethargic, slacked off/doesnt want to eat or drink, has slacked off laying or doesnt lay eggs...the majority of the time it is worms causing this problem and it is an infestation causing it. The chicken will eventually die from starvation because the worms are feeding off the nutrients that the chicken should be absorbing into its system.