As a way to protect themselves from predators chickens hide their symptoms of ill health until the very last minute, so it is really important to keep a look out for symptoms. As soon as you see a sign of ill health take action before it’s too late. Take your chicken to a good chicken vet and get the illness treated before it gets any worse.
Chickens can be in a lot of pain and discomfort, but barely let their carers see that there is anything wrong. This is a diagram of a hen that I give to new battery hen adopters. Many people are unaware of how subtle symptoms of ill health can be in chickens and this gives you an idea of what to look for. You should give your girls a visual check-over every day when you let them out of the coop. Ideally you should also pick them up regularly to feel for any strange lumps and bumps and check their weight.
One of the most simple things to look out for is a messy bottom. It is nearly always a sign that a hen has something wrong in the reproductive area, whether an infection that could be easily treated with meds or egg peritonitis or a tumour that requires more advanced treatment.
If you pick up symptoms early you have so much more chance of helping your sick hen to get healthy again.