Porcine circovirus (PCV) is a group of viruses. There are 2 strains: type 1 and type 2. Porcine circovirus associated disease is caused by porcine circovirus type 2 (PCV2).
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. 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.