Often referred to as PBFD . Here i shall just touch on the basics of the disease.There is much more detailed articles on the internet but this will give you a brief idea of the disease.

BFD is caused by a DNA virus that affects the cells of the immune system and those that produce the beak and feathers. The virus is a circovirus, which is one of the smallest viruses known to cause disease.
PBFD is extremely contagious.Large amounts of the virus, which can become airborne, are found in the droppings, contents of the crop, and the feather dust of infected birds. The feather dust is easily dispersed and can contaminate food, water, cages, clothing, and other areas of the environment.It has been suggested that the virus may be transmitted in utero from the female bird to the egg.PBFD is thought to be transmitted by inhalation or ingestion of the virus.
Acute Form: The peracute and acute forms most commonly occur in very young birds, and may begin with signs unrelated to the beak or feathers. Affected birds are often depressed and regurgitate due to crop stasis. They may develop a diarrhea-causing enteritis or pneumonia, and die without displaying any lesions of the feathers or beak. This is often called the peracute form of the disease. In the acute form, juveniles losing their down and developing feathers may have lesions on the feathers, including circular bands around the feathers which constrict the feather at its base. These feathers are often loose, break easily, may bleed, and are very painful.
Chronic Form: In the chronic form of PBFD, which is more common in older birds, the powder-down feathers are often the first feathers affected. The feathers are fragile and fracture easily, have constricting bands, may hemorrhage, and may be discolored, deformed, or curled. As the feather follicles are damaged, the bird will soon be unable to replace feathers, and the primary, secondary, tail, and crest feathers are lost. Bare skin is exposed, and the normal feather dust is not found on the body or the beak, where it normally accumulates due to preening. Feather abnormalities, often termed "dystrophic feathers," may not appear until the first molt after infection, which could be a period up to 6 months.

The beak may develop irregular sunken areas. Brown necrotic areas may be found inside the upper beak, and the beak may elongate, become deformed, and fracture. Secondary beak and oral infections often occur. In some birds, the nails can also be deformed or slough.

Mucus in the droppings, or a green tint to the droppings may occur. In some birds, the liver will be affected, and liver failure may be the cause of death.

Birds with the chronic form of the disease may live for months to years before dying of a secondary infection. This long period of illness in which the bird may be featherless, and gradually weakens can be very emotionally difficult for owners.
Diagnosis is via a Skin biopsy, surgical biopsy of feather and shaft, or PCR testing of blood, swab, and feather samples.
Some birds infected with the virus, test positive, but never show clinical signs. Other birds which test positive may develop an immune response sufficient enough to fight off the infection and test negative after 30-90 days. Therefore, it is recommended to re-test all PBFD positive birds 60-90 days after the initial testing was completed.
There is No known treatment. Experimental vaccines are being developed.Supportive care including good nutrition, supplementary heat (incubator), beak trimming, and treatment of secondary infections can be offered. The disease, however, is progressive, and very few birds recover

written by Tracy Craig