I recently had a bout with Fowl Pox in my flock of chickens. I had never had this problem and wasn't sure what was going on with them! Most of them came down with strange black spots on their combs, wattles and even a few of them had it on their legs. They looked droopy and their egg production went down by about 70%. This is very horrible when you depend on the eggs to pay for their feed bills and your egg customers don't get very many eggs!
I did a lot of research and didn't want to give them any antibiotics or medicine that would carry through to the eggs so I started putting a little simple medicine called Vet RX in their waterer and on their combs. It smells like Eucalyptus and mint and seemed to give them some relief, especially the ones that were having stuffed up noses and a hard time breathing. I also added Colloquial Silver to their waterer and that seemed to help them as well.
There are other natural remedies that could be used but those worked well for me.
My other tips to keeping chickens healthy are of course keeping their coops clean. Chickens are very messy birds who poop a lot! Adding clean bedding to their coop and cleaning it on a regular basis helps to keep them healthy. Keeping their food and water containers clean and free from poop also helps a lot!
Fresh food and fresh green treats and proper nutrition help to keep them healthy and laying well and growing well for the young birds. If they have access to be able to free range safely in a yard or area, they are going to be much happier and healthier than birds who are locked up in a small pen every day.
Chickens are also very prone to external parasites such as chicken mites. These will literally suck the life out of your flock. One natural way to combat these are to dust the chickens with sulfur powder. I like to do this at night when they are all tucked in bed. I dust each of them with the powder and make sure to dust the coop too. You might end up smelling a little like sulfur too but it is worth it to keep the birds happy and healthy.
You can also provide them with clean sand to dust in. This helps with the mite problem too. And it's quite entertaining to watch a chicken take a dust bath! One neat way to give them a dust bath is with an old tire. Fill it with play sand and watch them have at it. You could also add a little Diatomaceous earth to the sand which will help with any external parasites.
Hopefully some of these tips will help you keep your chickens healthy and happy and laying lots of eggs!