Animal circoviruses belong to the Circovirus genus of the Circoviridae family. Nowadays, only swine and birds were identified as circovirus hosts. Circoviruses have a single-stranded circular genome of approximately 2000 nucleotide long. DNA of these viruses possesses : (i) a nonanucleotide sequence essential for replication, flanked by inverted repeat sequences, a palindrome that has the potential to form a stem-loop structure and (ii) two major ORFs, located on the viral and complementary strands, which encode respectively the replication-associated protein (Rep) and the capsid protein (Cap). All the circoviruses described at the present time, except porcine circovirus of type 1, are associated with immunosuppressive or immunodepressive diseases. Histopathological lesions such as cytoplasmic inclusions of virus in histiocytic cells and T and B lymphocyte depletion in lymphoid organs, are commonly noticed. No medical prophylaxis of circovirus infections is currently available.