From the University of Florida:
MAREK'S DISEASE
Synonyms: acute leukosis, neural leukosis, range paralysis, gray eye (when eye affected)
Species affected: Chickens between 12 to 25 weeks of age are most commonly clinically affected. Occasionally pheasants, quail, game fowl and turkeys can be infected.
Clinical signs: Marek's disease is a type of avian cancer. Tumors in nerves cause lameness and paralysis. Tumors can occur in the eyes and cause irregularly shaped pupils and blindness. Tumors of the liver, kidney, spleen, gonads, pancreas, proventriculus, lungs, muscles, and skin can cause incoordination, unthriftiness, paleness, weak labored breathing, and enlarged feather follicles. In terminal stages, the birds are emaciated with pale, scaly combs and greenish diarrhea (see Table 2).
Marek's disease is very similar to Lymphoid Leukosis, but Marek's usually occurs in chickens 12 to 25 weeks of age and Lymphoid Leukosis usually starts at 16 weeks of age.
Transmission: The Marek's virus is transmitted by air within the poultry house. It is in the feather dander, chicken house dust, feces and saliva. Infected birds carry the virus in their blood for life and are a source of infection for susceptible birds.
Treatment: none
Prevention: Chicks can be vaccinated at the hatchery. While the vaccination prevents tumor formation, it does not prevent infection by the virus.
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I am
NOT an expert!!! and have no first hand knowledge of any avian diseases, apart from vent gleet (for which I am extremely thankful), but the age of your bird at 3 weeks suggests (to me, in my ignorance) that its not Marek's.
AE (Avian Encephalomylitis) strikes in your age, and shares paralysis as a final symptom (and sadly, the same fatal outcome for affected birds), but you didn't mention head tremors leading up to this, which are another of the disease's symptoms.
I'm going to defer tot he expertise of others, but early prognosis is not good.
I hope you will help me soon...
