However, there are a few additional things that you can do on a regular basis to promote good health.
The first of these is to ensure that your poultry housing is dry and well ventilated. In my opinion, many small hen houses are not ventilated well enough and need extra holes drilling around the top of them. Chickens do not like to be in a draft; but lack of ventilation encourages bronchial complaints.
It is an inescapable fact that if your birds are outside and in contact with wild birds, then they will get parasites and diseases from them. But if they have a strong immune system and are well fed and housed, they should, mostly, be able to fight disease off themselves.
I am not a vet or an expert in poultry ailments and this is really just a starting point. There are many books out there that specialise in the health and diseases of poultry and I would recommend reading one of them - Victoria Robert's 'Diseases of Freerange Poultry' is particularly good. Also, find a poultry-friendly vet before you actually need one, 'just in case'.
If your chickens are reluctant to go to bed at night and seem to be a bit off colour, it might be that you have them. They can live for a long time without chickens to feed off, so a second hand house might have them; or you might buy in birds with a small mite population that gets worse. You can check the birds' feathers to see if they have a mite or louse problem and you may see them around the bottom of their trousers, just at the top of their legs. You can get various generic 'mite and louse' powders or sprays and if you have an outbreak of mites you will need to dust or spray each chicken and disinfect all the nooks and crannies of the house where they hide. I tend to include a couple of table-spoonfuls of mite and louse powder with their clean bedding every month or so, as a preventative - this works well for me, as the birds like to dust-bath in their house. Creosote or a blow-torch are other very satisfying ways to get rid of mites. If you are going to use both, remember to do the blow-torching first to avoid setting the house on fire! Pay particular attention to under the roofing felt if you use it - it harbours the mites.
Other things many people do include putting a drop of 'frontline' cat/dog flea treatment under each wing every few months. This is NOT a medicine that is prescribed for chickens and it is also an organophosphate, which means that in large quantities it can cause cancer. Chickens don't live long enough for that to be a problem. But I, personally, wouldn't be happy to be eating meat or eggs from birds that had had it applied to them regularly. It is useful thing to know about if you are desperate, though.
The 'red mite' season seems to be the wet and warm periods of the year. Cleaning the house thoroughly, splooshing Jeyes Fluid about in all the cracks and crevices of the house and sticking vaseline around the ends of your perches - so the mites get stuck in it when they come out to feed on the birds at night - is my preferred method of control at the moment.
Other methods include smothering the legs with vaseline to suffocate the mites, various proprietary sprays and the VERY old fashioned method of painting on creosote, which I have read about in old books but never heard of anyone I know of still using. 'Frontline' spray is used by people sometimes in cases which seem to be difficult to get rid of, with the same reservations set out earlier in the 'red mite' section.
Some birds seem to be prone to scaley leg - the feathered-leg breeds in particular, because the feathers harbour the mites; but individual birds in a flock sometimes seem to have it all the time whilst their flock-mates are clean.
Mycoplasma is a very infectious disease of poultry that is spread by them sneezing, often in to their water. It is pretty common in all flocks as a sort of 'background' disease that comes out when the birds are stressed, overcrowded, a bit run down (from something like red mite, worms or scaley leg) or in poorly ventilated housing. As you can see from the photo to the right, the sinuses and face can get very swollen. The eyes sometimes get 'gummed up' or have clear, soapy bubbles in the corners and they can have a runny nose. They also exhibit a classic 'rattle' in their breathing and can 'gape' for breath, stretching the head and neck in the air and opening their mouth. There is also supposed to be a classic unpleasant smell about suffers.
There are various different variants of the disease, so they might not exhibit all the symptoms. Treatment is usually antibiotics - Tylan or Baytril are the most common and need to be got on prescription and either syringed down sufferers or put in their water. Respite Poultry Tonic, Herban and Collodial Silver are also treatments that can be used.