Recently thousands of dead and decaying pigs were pulled from rivers in Shanghai and Jiaxing, China. Apparently farmers dumped the animals into the water after the pigs became ill, and porcine circovirus was subsequently detected in the in pig carcasses and in the water. Porcine circoviruses are small, icosahedral viruses that were discovered in 1974 as contaminants of a porcine kidney cell line. They were later called circoviruses when their genome was found to be a circular, single-stranded DNA molecule. Upon entry into cells, the viral ssDNA genome enters the nucleus where it is made double-stranded by host enzymes. It is then transcribed by host RNA polymerase II to form mRNAs that are translated into viral proteins. There is some evidence that circoviruses might have evolved from a plant virus that switched hosts and then recombined with a picorna-like virus.