Circovirus...

It is known that both PBFD and CAA cause disruption of the tissues comprising the lymphoid system. This system (including the spleen, thymus, Bursa of Fabricius, and both gut- and bronchial-associated lymphoid tissues) houses cells that are responsible for most immune functions of the body: making antibodies, recognizing disease organisms, processing antigens, and so forth. Because CAA and PBFD viruses harm the lymphoid organs, birds having these diseases are "immunosuppressed". That is, these viruses cause a form of acquired immunodeficiency (like human AIDS). One result is that secondary infections with other viruses, bacteria and fungi are common among birds with PBFD and CAA, even when birds are vaccinated against the secondary problems. In other words, Circoviruses compromise the birds' ability to benefit from vaccination!