Metabolism means simply the utilisation of food and its effects within the body.
Actual infections are few and far between.
You have to remember, that what we as breeders have done to the budgerigar is not at all commendable. They have been produced because we have selected for specific features which are visually satisfying to us. At the same time we have unwittingly selected other matters which we cannot see. Many of the problems we now encounter in our aviaries today are the result of the unknown quantities that we keep pushing into our studs. Today we fuss over them and those which would normally have died in the wild, survive to perpetuate more problems.