The Circoviridae family is an extraordinarily diverse group of small, single-stranded circular DNA viruses that includes cycloviruses (genus Cyclovirus) and circoviruses (genus Circovirus), which are commonly detected in human stool and blood, and also in environmental samples [43] [44] [45] [46] [47] [48] . Some circovirus species, such as beak and feather disease virus and porcine circovirus 2, have been associated with disease in bird and pig hosts, respectively, but the pathogenic potential of circoviruses in humans remains unconfirmed [63, 64] . The circovirus-like sequences reported here were detected in nucleic acid libraries prepared from acute human serum and were most closely related to circovirus-like viruses (Figure 3 ), which were first reported in environmental samples and in bats [45, 48] . We were unsuccessful in recovering a full genome sequence corresponding to any of the circovirus-like sequences, and it has not yet been possible to prove that these sequences were not an environmental artifact introduced during sample preparation. It is also possible that these sequences derive from other organisms, such as Giardia intestinalis or Entamoeba dispar, whose genomes encode proteins that share amino acid similarity with circovirus replicase proteins ( Figure 3) . Furthermore, it has yet to be established whether circoviruses are capable of replicating in humans. Pending additional screening and serologic studies, the detection of circovirus-like sequences from human serum should be interpreted with caution.