Porcine circovirus 2 is the major predisposing factor for infectious diseases of weanling piglets, causing a big impact on the economy of pig production. The virus causes lymphocyte depletion, immuno-suppression leading to disease, wasting and ill-thrift of weanling piglets. Surprisingly, it is the smallest independently replicating mammalian virus, causing a devastating impact on the health of a large vetebrate.