Porcine circovirus (PCV) is a group of four single-stranded DNA viruses that are non-enveloped with an unsegmented circular genome. They are members of the genus Circovirus that can infect pigs. The viral capsid is icosahedral and approximately 17 nm in diameter. PCVs are the smallest viruses replicating autonomously in eukaryotic cells. They replicate in the nucleus of infected cells, using the host polymerase for genome amplification. PCV-2 causes Porcine circovirus associated disease or postweaning multisystemic wasting syndrome (PMWS). An effective vaccination is now available. Fort Dodge Animal Health (Wyeth) launched the first USDA approved vaccine in 2006, containing an inactivated virus (ATCvet code: ). Classification Three strains of PCV are known as of 2018: PCV-1 (first identified in 1974) readily infects, but is not known to cause disease in swine. PCV-2 (first isolated in 1997) causes PMWS, which over time results in significant depletion of lymphocytes; postmortem examination of diseased animals reveals enlarged lymph nodes and abnormal lung tissue. However, viral infection by itself tends to cause only mild disease, and co-factors such as other infections or immunostimulation seem necessary for development of severe disease. For example, concurrent infection with porcine parvovirus or PRRS virus, or immunostimulation lead to increased replication of PCV-2 and more severe disease in PCV-2-infected pigs. PCV-3 (first described in 2015) causes a wide range of problems, and may be widespread among pigs. PCV-1 and PCV-2 show a high degree of sequence identity and a similar genomic organisation; nevertheless, the basis of the distinct pathogenicity has not yet been unravelled. The organization for PCV-3 is similar, but the sequence identity is much lower. Genome PCV's genome is one of the simplest of all viruses, requiring only a capsid protein (ORF2) and two replicase proteins (ORF1) in order to replicate and produce a functional virus. Due to the simplicity of PCV, it must rely heavily on the host's cellular machinery to replicate. The origin of replication is located on a small octanucleotide stem-loop that is flanked by palindromic repeats, with the ORF's being located head-to-head on both sides of the Ori. Specifically, ORF1 is located clockwise and ORF2 is located counterclockwise of the Ori. The two replicase enzymes that are created from ORF1, Rep and Rep', are conserved between the two types of PCV, and are part of the early phase of the virus. The replicases differ in that Rep is the full ORF1 transcript of 312