Beak & Feather Disease (PBFD)/Circo viruses target the precursors of T lymphocytes & macrophages resulting in lymphocyte depletion, thus impaired immunity. This impaired immunity allows the invasion of super- or co-infecting agents (viruses, bacteria, funguses, parasites), which can occur simultaneous with circoviruses induce illnesses and increases severity of circovirus disease.
Examples are French molt where Polyoma virus quite often occurs concurrent with Circo virus; Parrots with PBFD developing respiratory infections. Quite often death is due to co-infections rather than the Circo virus.
