The only way to know for sure what the issue is,is a confirmed diagnoses from either a necropsy or a vet. There is really no point in guessing as this can have disasterous results.
The only thing you can try is treating for the disease/virus that most matches your birds symptoms,but again this is just guess work.
The only thing you can try is treating for the disease/virus that most matches your birds symptoms,but again this is just guess work.