PRRSV causes abnormal development of immune function. This is believed to allow the virus to persist in carrier animals that remain relatively healthy. Such disregulation of the immune is a common feature of many viral infections. Good parasites exist by only partially disabling their host’s immune system which in turn allows them to survive and be transmitted without causing such wide-spread damage that the host species is eliminated. Until a thoroughly dependable vaccine is developed through genetic engineering, detection of the carrier status of animals is likely to be an important safeguard that can be used in connection with proper management practice. Whether the carrier state can be reliably detected by assays that depend on serum antibodies in animals that have recovered from a hyperplastic condition, needs to be addressed. [J. E. Butler, The University of Iowa; [email protected]]