Porcine circovirus (PCV) has been classified as a member of family Circoviridae in 1993. Cases of PCV in sucking piglets, nursing pigs and fattening pigs were reported in 1997 in Taiwan. An investigation showed that out of 204 pigs' specimens submitted by Livestock Disease Control Centers and farmers, 60% were co-infections of PCV2 with Salmonella. So far, no treatment and vaccine are available for PCV2 infection. References elsewhere showed that PCV is highly resistant to commonly used disinfectants. Data analyzed by this institute and Bureau of Animal and Plant Health Inspection and Quarantine indicated that respiratory disease of pigs has changed from bacterial infections (e.g., Actinopleuropneumonia) in early stage (1987 to 1992) to complicated forms (1999 to 2003) as porcine reproductive and respiratory syndrome and PCV2 viruses were introduced to Taiwan. Viral disease complicated with secondary bacterial infection would be found mostly in particular among the pig's cases. This project will present us an opportunity to study the pathogenesis and pathogenicity of PCV and hopefully to establish a control measure for pig farms.