The information below is only our experience. We are not veterinary surgeons so please bear this in mind and seek help from a vet if you are at all worried.
As far as diseases go aspergillosis is one which is heard of fairly often when discussing pheasant health. It is caused by the aspergillus fungus which is widespread in the environment. It thrives in heat and humidity and will easily become airborne when disturbed. Most organic materials will host the mould as it is part of the rotting process. It is therefore important to keep the aviary bedding clean and dry. Damp food will very quickly go off and allow the fungal spores to spread rapidly. Hay and sawdust are often the cause of problems and if using straw be sure it is not mouldy as it is also a common source. Wood shavings are normally a safer choice for bedding.
Signs a bird may have aspergillosis are heavy breathing or gasping. The pheasant may make a clicking noise or, if the airsacs are damaged, the chest may swell and deflate like a balloon in time to it's breathing. Other signs can be a nasal discharge or watery eyes. Young chicks may die without warning so check bedding is kept dry and clean. Although spores are usually ingested aspergillosis can be passed through eggs too so once again it is important to keep nesting areas as clean as possible.
The disease can kill so it is vital it is diagnosed quickly and the bird is moved and the living area cleaned thoroughly. Using a pet safe disinfectant with a fungicide such as Virkon S should help.
If any news is good news it's that it's not normally spread from bird to bird. The bad news is there's not much available in the way of treatment for pheasants with aspergillosis but give the birds extra heat and clean water with added bird tonic to boost it's immmune system. You can check with the vet in case they can prescribe a fungicide for your bird but an antibiotic will not have any effect on the fungus and usually do more harm than good by killing good bacteria that will help the animal's immune system fight off illness.
Prevention is the best course of action and by providing a clean environment, a good diet and keeping your birds stress free they will throw off any threat to their health easily, as they should.
I have included mycoplasma alongside aspergillosis due to the similarities of their symptoms however Mycoplasma is a bacteria (not a fungus) which attacks the respiratory system. Unfortunately it does not always respond well to a treatment of antibiotics. If diagnosed early Tylan can work but if the bird does not improve quickly it may well be better to cull the bird humanely as it can spread throughout all your birds. It can be spread via any eggs laid by infected birds. It can also be carried by birds not showing signs of illness themselves. Younger birds are more susceptible to Mycoplasmosis than mature birds.
If a bird is sneezing or snicking (sounds like a cross between a cough and a sneeze) it could have mycoplasmosis. Other symptoms which might be seen are watery eyes or nasal discharge and inflammation around the eyes and sinus area. Lameness can be another possible sign and there is a chance you will notice a sweet sickly smell in the aviary. As these symptoms can be symptoms of other infections it is best to take a sample of any discharge to a veterinary surgeon to be tested.
By not overstocking birds, giving them the best diet possible and only using strong, healthy birds for breeding you will give your birds a good immune system which will be able to fight off disease before it can take hold.